What computer operating system is involved here?
Did you ever install Premiere Elements 11 on your computer?
If so, then go to the Adobe web site http://www.adobe.com, sign in with your Adobe ID and Password, go to the Search field (top right of the web page) and type in My Products. Follow the My Products link to the list of your registered products. If you find your Adobe Premiere Elements 11 listed there, click on it to reveal the serial number.
Please let us know the outcome.
Add On...remember, Adobe will not let you use the purchased activated serial number on more than 2 of your computers (and then, one at a time).