Premiere Elements 12 on iMac (what operating system?). Have you updated 12 to 12.1 yet using an opened project's Help Menu/Updates?
What specific Canon camera recorded this .mov file (model/settings)? Did you modify the original Canon recording in any way before you tried to import it into Premiere Elements 12 Mac? The following describes THM file as essentially an informational file. Although it may contain the name of the video file it represents, it appears that it does not have the format of the video.
Important: Do you know if the .mov file that you are trying to import has the codec = H.264 or Apple Intermediate Code?
The Media Info video and properties program should give you that information if you do not already have it.
Just be careful downloading and installing this program since it has been know to travel with unwanted baggage.