The Nucleus, from Solid State Logic, is a 16 fader Mackie Control device that includes many buttons, separate meters, two LCD displays and other features. The device is not cheap (around US$5000 at the time of writing), and has some design features (or lack thereof) which some Ardour developers find questionable. Nevertheless, it is a very flexible device, and makes a nice 16 fader surface without the need to somehow attach an extender to your main surface.
Your Nucleus comes complete with a number of "profiles" for a few well-known DAWs. At the time of writing it does not include one for Ardour (or related products such as Harrison Mixbus). This is unfortunate because although Ardour could easily be used with any of the existing profiles, one thing they all have in common is a remarkably large of buttons not assigned to Mackie Control functions. This means that using one of them will "waste" the buttons, a resource that the Nucleus is not particularly rich in (compared with some other Mackie Control devices).
Unlike most Mackie Control devices, the Nucleus uses an ethernet connection to send and receive the MIDI messages that make up the Mackie Control protocol. Specifically, it uses a technology called "ipMIDI" which essentially "broadcasts" MIDI messages on a local area network, so that any connected devices (computers, control surfaces, tablets etc.) can participate.
All other DAWs so far that support the Nucleus have chosen to do so by using a 3rd party MIDI driver called "ipMIDI", which creates a number of "virtual" MIDI ports on your computer. You, the user, tells the DAW which ports to connect to, and ipMIDI takes care of the rest.
Ardour has builtin ipMIDI support, with no need of any 3rd party packages, and no need to identify the "ports" to connect to in order to communicate with the Nucleus. This makes setting it up a bit easier than most other systems.
Unless ... you already installed the ipMIDI driver in order to use some other DAW with your Nucleus. If ipMIDI is configured to create any "ports", it is not possible for Ardour's own ipMIDI support to function. We decided to offer both methods of communicating with your Nucleus. If you regularly use other DAWs, and appreciate having ipMIDI permanently set up to communication with the Nucleus - that's OK, you can tell Ardour to use the ipMIDI driver you already have. But if you're not using other DAWs with the Nucleus (and thus have not installed the ipMIDI driver), then you can ignore the ipMIDI driver entirely, and let Ardour connect directly with no configuration.
This is usable only on computers with no 3rd party ipMIDI driver software installed and configured. If you have the OS X or Windows ipMIDI driver from nerds.de, it MUST be configured to offer ZERO ports before using this method.
Open Preferences > Control Surfaces. Ensure that the
Mackie protocol is enabled, then double-click on it to open the
Mackie Control setup dialog.
Ensure that the device selected is "SSL Nucleus". The dialog should show a single numerical selector control below it, defining the ipMIDI port number to use (it should almost always be left at the default value of 21928).
Communication is automatically established with the Nucleus and you need do nothing more.
If this does not work, then make sure your network cables are properly connected, and that you are not running other ipMIDI software on the computer.
This is usable only on computers with 3rd party ipMIDI driver software installed and configured for (at least) 2 ports.
Open Preferences > Control Surfaces. Ensure that the
Mackie protocol is enabled, then double-click on it to open the
Mackie Control setup dialog.
Ensure that the device selected is "SSL Nucleus (via platform MIDI)". The dialog should show four combo/dropdown selectors, labelled (respectively):
Communication should be automatically established with the Nucleus.
If this does not work, then make sure your network cables are properly connected, and that you are running the approprate ipMIDI driver and have configured it for 2 (or more) ports.
You might be reading this part of the manual seeking some guidance on whether the Nucleus would make a suitable control surface for your workflows. We don't want to try to answer that question definitively, since the real answer depends on the very specific details of your workflow and situation, but we would like to point out a number of design features of the Nucleus that might change your opinion.