Welcome to the avast forum,
If you already got the license of avast Free edition, then you can input into the avast GUI.
1. Start - avast GUI will show in memory scan (v.4.
2. Initially avast will ask you to input your license, but if you already open avast before then you can click "License Key" when avast on memory scan progress
Please come back to us, if you still have any issues regarding to license key of avast.