Free does not come with a license key. All you need to do is register via the GUI.
For AIS you received it via email. Check your download folder. From the avast GUI "click" the "insert license key" to activate.
If you do not want AIS you need to uninstall AIS then install the free version.
1) download and save to HDD the version you desire from http://forum.avast.com/index.php?topic=108170.msg856869#msg856869
2) download and save to HDD the uninstall utility http://www.avast.com/uninstall-utility
3) reboot into "safe mode" by pressing "F8" repeatedly
4) run uninstall utility from step #2 for each version of avast you have
5) reboot into "normal mode"
6) install the version of avast you selected from step #1
7) reboot and register via the GUI