Have you updated Premiere Elements 12 Mac to the 12.1 Update? If not, please do so and let us know the outcome. In Windows, you do that
using an opened project's Help Menu/Update. I am assuming you would do the same for Premiere Element 12/12.1 Mac.
Based on history, Adobe is not expected to revisit version 12/12.1 now that Premiere Elements 13 has been released.
Adobe had been offering a "changing" of 13 for 12 if 12 was purchased within 30 days of the release of 13. Premiere Elements 13
was released September 24, 2014. If you purchased Premiere Elements 12 Mac a few months ago, it would appear that you would not be eligible for this changing of programs. But, we are not Adobe. Rather user to user.
Did you purchase it from Adobe as an online download? If from Adobe, you might want to contact Adobe via its Adobe Chat (Orders group) to confirm your situation. We can suggest a link to try if you want to explore that idea.
We will be watching for further developments.
Just an add on...an associate of mine has told me about the Photoshop Elements 12 Yosemite issues that are in discussion
at the Adobe Photoshop Elements Forum and other online forum.
At this point, I do not know if Premiere Elements and Photoshop Elements share common problems with regard to Yosemite.
You might want to take a look into this on the Photoshop Elements side of things...