Moving this discussion to the Premiere Elements forum.
1. Latest version of QuickTime installed?
2. Running the program as Administrator - all privileges in place?
3. Video card/graphics card drive version up to date according to the web site of the manufacturer of the card?
4. Does you iMac come with any particular security program?
I am strictly an Elements Windows users, so you will need to translate that following to Mac
Delete the Adobe Premiere Elements Prefs file...perhaps located in Mac, Libraries/Users/User Name/Application Support/Adobe/Premiere Elements/12.0/
and in the 12.0 Folder may be the Adobe Premiere Elements Prefs file that you delete.
Let us start here and then consider what next.