Before you worry about potential conversions, let's make sure you were importing properly.
Go to the file in iPhoto and right-click/Reveal in Finder. Make a copy of the file (Command-C) and paste it on the Desktop (Command-P).
Now, in iMovie, use FILE/IMPORT MOVIE and navigate to this file on the Desktop. Does it import?
If not, read on...
MOV is a container, not a codec. iMovie will only import files that it can edit. You will want to convert it to something, but probably not DV, since if you had HD video you would lose all the HD resolution and get it converted to standard definition.
Open your MOV file in QuickTime Player. Then hold down the Command Key while typing the letter i. This will open the Inspector. Tell us the dimensions of your clip and everything that is listed next to the word format, and we can give you advice on how to convert.