it depends... if it's a home screen, unzip it on the pc, and copy it to the Q. if it's a application 1) a .cab file can be installed from/on the Q (copy it to the Q and execute it with the File Manager 2) if it's an .exe execute it from pc with the Q synced up/connected and it will install.
and check this:
http://www.everythingq.com/moto-q-new-user-guide/