In all recent versions of OS X, it has been critically important to delete all preferences before running PSE.
If you have once run PSE it is necessary to also delete the saved application states.
So you would need to go to your user library and delete:
com.adobe.PhotoshopElements.plist
Adobe Photoshop Elements < version > Paths
Adobe Photoshop Elements < version > Settings
com.adobe.Elements Organizer. < version >.plist
com.adobe.ElementsOrganizer< version>.plist
and any lockfiles with the same names, then go to Saved Application States (at the same level as preferences) and delete any for PSE, then repair permissions and try again.
However, I will say that I am not running Yosemite (and don't intend to any time soon), but from the folks I knew who were in the Apple beta, it breaks many, many existing programs.
I don't know if the user library is still hidden in Yosemite, although I would guess so, given how many other useful thing are now hidden. If it's like previous versions of OS X click the Go menu in the Finder and hold down the Option key and it will appear below the little house for your user account.