The company who makes the Clifford system is DEI, or Directed. With this link you can input your model # and download a pdf. with all the information you need. Directed - Owner's Guides
DEI installs a relay which is located under the dash below or near the steering column (exact location picked by the installer). It is easy to identify- it is the only relay which intercepts a wire from the ignition switch. Get a flashlight, and find the small bundle of large guage wire coming from the ignition switch. You should find that a wire has been cut in half and run through a relay sometimes its black/white. If you remove the two cut halves from the relay and restore the wire (or its connection temporarily) the car will start regardless of the starter disable.
As far as getting in, unlock the car with the key, try activating the valet feature. Their will be some form of momentary switch to push. Sometimes in good installs the installer will wire the valet or starter kill interrupt inline with an existing momentary function of the car, ie (headlight high beam flash). You could also pop the hood and pull the fuse, unless the vehicle has a back up battery, then you will need to locate it and pull it out. I would disconnect the audible alert until you get it figured out. It could be as easy as unplugging the siren or difficult as finding the horn fuse/relay and disabling that system.