You should not need to do anything special in the Extended Settings menu for it to work.
1) Did you confirm the trunked system object for MARCS is set up for MOT 800/900?
2) Do you have the Group Wildcard object defined in a scan group and linked to the trunked system object set up for MARCS?
I am sure it is probably something that is being overlooked. If you used WIN500 I would make you a test file and send it to you. Are you a Premium member of RR? If so, you should be able to download the data directly from the web site to your ARC500 app which may help solve the problem as well.
Just for fun, if you hit the "tune" button, enter one of the control channel freqs, and hit enter do you hear the data stream?