If you are trying to use Kontakt as a standalone, you need to go into Options (the gear icon at the top of Kontakt's window) and there into "MIDI". There you hopefully find Seaboard Block and assign it to one of Kontakt's ports - normally you choose port A. Then it will be listed as "Port A (Seaboard Block)" in the MIDI channel list of Kontakt's instrument headers.
Mind that some Kontakt instruments can only listen to a single channel, even if set to Omni. For those you'll need to set Seaboard Block to Single Channel mode in Dashboard. If you want to benefit from per-note-expressions you most likely need to create a separate instrument for each voice, each set to listen to it's own MIDI channel. I'm saying "most likely", because there may be Kontakt instruments that can do polyphonic expression in a single instance.
Hope this helps. I think there is a "How to set up Seaboard with Kontakt" page somewhere here on ROLI's site, but couldn't find it quickly. You also should be able to find a video tutorial about it on, say, YouTube.