Motion JPEG on an iMAC

How do I convert a .mov to "Motion JPEG" which is the only format a digital photo frame by Trutech will let me upload video in?

You can purchase a QT Pro registration (about $30) and export your movie directly from the QT Player or you can download an application like MPEG Streamclip (free), open your movie in it, and use the "Export to QT" File menu option and select the Motion JPEG A or B codec.

Motion JPEG codec is nearly loss-less and could create very large file sized videos.
You should consider exporting at a small dimension (no more than 320X240) and at a very reduced frame rate. If your source files at 30fps you may want to use 10fps (or less) on your exports to help keep the file size low. You can also adjust the compression to help reduce the file size.