There is an article about how YOU do this in the FAQ pages
Premiere Elements FAQ List (2 pages of FAQ as of September 2013) http://forums.adobe.com/community/premiere_elements/premiere_elements_faq?view=all
The Tutorial Links Page http://forums.adobe.com/thread/1275830 may also help
Premiere Elements TIPS List (6 pages of Tips as of September 2013) http://forums.adobe.com/community/premiere_elements/premiere_elements_tips?view=all
When you say that you still have the Adobe watermark on your images and video clips, are you specifically talking about those images and video clips within a given project (project.prel file) OR are you referring to exports from the tryout product (file saved to hard drive or burn to disc)?
The Adobe watermark cannot be removed from an export of the Premiere Elements tryout. It can be removed from a project.prel file created in the tryout version. I have found it is an automatic process although some have reported the classical fix worked for them, that being, deleting the rendered files. If you were dealing with your issue at a project.prel level, did that deletion of the rendered files work for you?
If not, this brings up a possibily interesting twist in this FAQ since you are working with a project.prel created in another version before it was opened in Premiere Elements 12. I have always looked at a project.prel created in the tryout not one created in another version and opened in the later version.
We will be looking forward to your results.
a. Premiere Elements 12 Expert workspace Timeline Menu/Delete rendered files
b. Going to the Adobe Premiere Elements Preview Files Folder and deleting the rendered files there.