You are in a good place with that purchase of the boxed version of the product which gives you an installation disc for Windows and an installation disc for Mac.
That type of purchase typically comes with one purchased serial number for Premiere Elements and one purchased serial number for Photoshop Elements which can be used for the Mac install from its disc or the Window install from its disc.
And, for a given product, the Adobe License allows you to use the same purchased serial number on no more than 2 of your computers. But, the Adobe License Agreement goes on to state, you are not supposed to run both at the same time.
After all of the above, I think that you will be OK
1. Important. Deactivate each Photoshop Elements 12 and Premiere Elements 12 from the Windows computer using Expert workspace Help Menu/Sign Out. You do not have to uninstall them if you do not want to. Deactivate is not the same as uninstall.
2. Then use the Mac disc to install Premiere Elements 12 and Photoshop Elements 12 with the Premiere Elements 12 serial number and Photoshop Elements serial number that came on the box label. These should be the same serial numbers that you used on Windows.
Please let us know the outcome. If any questions or need clarification, please do not hesitate to ask.
Add On...Do you realize that Premiere Elements Windows projects cannot be opened in Premiere Elements Mac and vice versa?