Do you already have a working Dashboard implementation running on your computer? If not, check out the information provided in the whitepapers section,
especially this one.
Once you have that up and running, you have a few "user bytes" you can send data back through, but that limits you to 4 bytes of information. But the Dashboard Spec also sends back information about all the PWMs and the Relays and such. So if you need to send back more data, you can just stick it in some unused PWM variables and read it off on the other side.