After receiving your license file, you just need to double-click to open it and your license will be inserted into your program automatically.
Alternatively, save the license file on your computer, open the avast! Pro Antivirus 5.0 or avast! Internet Security 5.0 interface and click on the 'MAINTENANCE' tab.
Next click on 'Subscription' and then on 'Insert license file'. A dialog window will open where you can browse your computer to locate your license file.
Once you have located it, double click on it and it will be automatically inserted into the program.
Hope that helps. Welcome to the forum.