Did you run load.sh first, to make sure you're on the right driver? Also, airmon-ng doesn't properly work with the version of aircrack-ng in the repos, because it conflicts with installing iw, and iw is apparently necessary for airmon-ng working right. In otherwords, I don't know all the technical details.

Do this to put your card into monitor mode if airmon-ng doesn't work:

Notice that the first and third command are iFconfig, the second one is iWconfig. If you use ifconfig instead of iwconfig it'll just give you an error. I never used iwconfig instead of ifconfig, so I'm not sure what it does. Also, I'm pretty sure you need to be root to do it.