I don't know about the specifics of any of your gear. Basically, you'll need to:
1) assign the track in your DAW to one of the interfaces outputs.
2) That output needs to go to the input of the hardware (API etc).
3) The output of the hardware then needs to go to an input on your interface and record to a new track.
DAW track --> output --> hardware input : hardware output --> interface input.
or if you wanna use more than one hardware...
DAW track --> interface output --> hardware input : hardware output --> to other hardware input --> other hardware output --> interface input.
make sense?
Also you'll need to pay attention to gain staging (volumes) so nothing is clipping, but the analog side is getting as much volume as it needs. Most DAWS -18 is the hardware 0 (in very general basic terms). So I usually make sure my track is around -18 and not peaking much above it. That way you won't clip your hardware. When the signal is coming back in from the hardware, I make sure it's not clipping if there is a meter on the hardware, if not I try to keep the DAW as close to -18 as possible.
ok, that's a lot, and I hope it wasn't too confusing. Basically make sure outputs go to inputs and your volume isn't too high. Got it?