The email you received should contain an attachement called "license.avastlic" -right?
Save that attechement to disk, e.g. to "My Documents".
Doubleclick on that saved file - done.
If that does not work:
Open Avast by clicking on the red ball with "a" in it in your system tray.
Go to "Maintenance" tab, "Subscription".
Click on "Insert License File" and navigate to the place where you have saved it.
Click "OK".
Done.
If that won't do also, we need detailed information about the problem.
Please come back here and give us input.