Although at this time Ardour does not support MIDI sequencing, it does support a fairly rich set of interactions via MIDI with other devices. In particular:
Ardour can function as MIDI Time Code (MTC) master or slave
Ardour can control or be controlled by other devices using MIDI Machine Control (MMC)
Ardour can bind all gain faders, panners, mute/solo/rec-enable buttons and all plugin parameters to be controlled by MIDI Continuous Controller (CC) or Note On/Off messages.
Ardour can send MIDI "feedback" whenever gain, pan or plugin state changes, so that external motorized control surfaces can reflect parameter changes caused by automation etc.
Ardour does not attempt to discover what MIDI ports exist on your system. This is a complex issue, and on systems like Linux and OS X that permit virtual ports to be created at any time, it is not trivial to get right (although future versions of Ardour may try).
Instead, the MIDI ports that are available for Ardour to use are
defined in your ardour.rc file. These port
definitions are not session specific, on the assumption that your
system's MIDI hardware probably doesn't change much from session to
session. The default version of this file contains a single port that
can be used for inter-application MIDI routing as well as MIDI I/O to
whatever physical MIDI ports might be available on your computer. In
many cases, you will not need to change them.
When you first use Ardour, the
ardour.rc file that you will have contains a
single port definition. It defines a port that is almost guaranteed to
be usable on your system ((Linux/ALSA users may need to ensure that
the snd-seq kernel module gets loaded - many
distributions do not do this by default)). This port is a "virtual
port" it isn't actually a hardware MIDI port, but instead is a
software port that can be connected to other software ports or to
whatever hardware MIDI ports you have (see
Section 1.2.4, “Making Connections”).
To define additional ports, find the line in
ardour.rc that looks roughly like this:
<MIDI-port tag="hw:0" device="/dev/snd/midiC1D0" type="alsa/raw" mode="duplex"/>
On OSX/CoreMIDI it would look more like:
<MIDI-port tag="coremidi" device="ardour" type="coremidi" mode="duplex"/>
You can then add another line right after it that looks similar but contains a different port definition.
You will see there are 4 pieces of information required to define a MIDI port for use within Ardour. Your port definition
This is just a name of your own choosing. It is how the port will be referred to within Ardour. You could use a name that describes what is plugged into the port (e.g. "1600x", "Novation"), or a name that describes the computer device/system that provides the port (e.g. "HDSP", "Sequencer"), or a whimsical name of your own choice (e.g. "bowtie", "merlin").
This is an operating system specific identifier that defines what kind of port this is. It can be set to one of three values:
alsa/raw - the port corresponds to a physical
MIDI port that is accessed directly without involving the ALSA
MIDI routing subsystem.
alsa/sequencer - the port is a virtual port
that can send and receive MIDI data via the ALSA MIDI routing
subsystem.
coremidi - the port is a virtual port that
can send and receive MIDI data via the CoreMidi
inter-application MIDI routing subsystem.
This is an operating specific and MIDI subsystem-specific name that actually identifies the device to be used for MIDI I/O.
for a raw ALSA hardware port, it is the name of the device file
corresponding to the hardware MIDI port. A typical value might
be /dev/snd/midiC0D0/.
for an ALSA sequencer port, it is an arbitrary name for the port that will appear as part of the ALSA MIDI routing system
for a CoreMIDI port, it is an arbitrary name for the port that will appear as in any CoreMIDI port selection dialogs.
This specifies whether the port is available for input, output or both:
read - the port is available for input only
write - the port is available for output only
duplex - the port is available for input and
output
You should probably always use duplex here. It is
rare to need to open a port for unidirectional communication only.
To remove a MIDI port, simply delete the line in your
ardour.rc file that defines it.
If you use actual physical hardware MIDI ports, then establishing connections to other MIDI equipment is simply a matter of connecting cables correctly. However, if you use "virtual ports" such as those offered by the ALSA router/sequencer or CoreMIDI, making connections is slightly more involved.
Ardour does not come with any way of establishing MIDI connections from/to other software MIDI ports. This is a difficult task to get right, and Ardour may offer something in the future. For now, you need to use an external tool that is dedicated to this purpose, much the same way you would use a patchbay (audio/MIDI) with physical equipment.
qjackctl (the same program that is recommended for controlling JACK) also includes an excellent MIDI connection manager. You could also use command line tools such as aconnect.
On OSX/CoreMIDI you need to connect the MIDI ports with a patchbay tool such as the excellent MIDI Patchbay from Pete Yandell