The Mac version of Premiere Elements MAY open the .prels from the PC version -- but no guarantees. The PC version includes features that the Mac version does not. So it will probably work.
As for loading the program onto a Mac, it depends on whether you downloaded the program or bought the boxed set. The boxed set includes discs for both PC and Macs. The download is either Mac or PC, and I don't believe you can use the same serial number on either download.