Not sure what you mean by mock up...some sort of graphics??
Assuming it is a graphic of some kind, what format is it in?
I will further assume that you will edit the graphic in Photoshop or Photoshop Elements and save it to a still file format supported by Premiere Elements 11. Then what is your intended export of the Premiere Elements 11 Timeline? Do you plan to incorporate it into a movie?
Please see the Premiere Elements Supported Import Formats Adobe document already given you, and then we can consider project strategy.