If you're talking about using it on the front end to compress/limit live inputs before they get recorded, you'll just insert it between the preamp and the interface and you're done.
If you're talking about processing existing tracks:
- Give the compressor it's own stereo I/O loop on the interface.
- Right-click the FX bin of a track, choose External Insert
- Set the I/O accordingly, and click the Delay button to have SONAR calculate the latency (or it will do it automatically the first time you start playback).
- To render the compression to an existing track, you have to bounce with Fast Bounce disabled so the external gear can process the audio in real time.