It appears that you are trying to edit all the contents of one folder, OR one placemark that contains <MultiGeometry>. If you are sure you are only trying to edit one placemark, and you get that error, then it contains <MultiGeometry>. If this is the case, then you cannot use GE to edit it. You must use a text editor, or some other program.

Hope this helps.

p.s. Could you post the specific file that you are a having a problem with?