Download the trial version and activate it with your serial. Legacy versions can be obtained using this method:
Contact support by phone or web chat, if you have exceeded your activation limit and need to have it reset.
If you have the most recent version of Elements you can download the trial from the Adobe product page and use your serial number to activate it.
If it is an older version, but not older than version 8, you can find the trial version at the site linked below and, again, you can use your serial number to activate it.
For the site linked below you need to follow the Very Important Instructions section in order for the download to work.
Photoshop/Premiere Elements 8:
Photoshop/Premiere Elements 9:
Photoshop/Premiere Elements 10:
Photoshop/Premiere Elements 11: