Adobe allows you to install the purchased serial number for your Premiere Elements 11 version on no more than 2 of your computers. So, if you have a 3rd computer and want to use the same product (same purchased serial number) on the 3rd computer, you need to deactivate the product on one of the other 2 computers. Deactivate is not the same as uninstall.
In Premiere Elements 11, deactivation is done in an opened project via Help Menu/Deactivate. If uninstall is wanted additionally, then the uninstall is done in the usual Control Panel manner.
I am assuming here that Windows computers are the only kind involved here? Please confirm. Are you working from installation files from an online purchase or from installation disc from the product purchased in boxed packaging?
From what I understand of the Adobe Licensing (you can verify with Adobe - we are not Adobe), you cannot/should not be using the two activated ones with the same purchased serial number at the same time.
Please review the above and then let us know if any questions or clarification needed.