Problem: In your finder an MP3 or picture file will be grayed out.
Solution: If your using the latest version of iLife you should be able to select the appropriate type (either Photos or Video) then drag the file the clip viewer.
If your using version 2 of iMovie or would like to import directly. Lennart was kind enough to write a few Apple Scripts to help you out for both OSX and OS9.
What
does Apple say about adding a song to a video?
I like to read the MP3 file into QuickTime Pro trim and edit in that program then when it is just the way I like it . I export the file to an .AIFF file. iMovie will import these files also.
What APPLE has written up about permissions and about repairing permissions
Set Type/Creator to JPEG/ogle. The REAL issue here is to understand the history the Mac file system. Originally, the name of a Mac file was never used as an indication of what type of file it was. (And still isn't!) There was no such thing as a file extension (.JPG, .MP3, etc) because users just named files ANYTHING they wanted. Therefore, the Mac OS needed some way to distinguish between the different kinds of files. In order to do this, the Mac OS kept track of 2 pieces of information about each file...a Creator and a Type.
It was not as common for Mac users to share files with non-Mac users, so it didn't cause much of a problem. Of course, when a non-Mac file is copied to a Mac system, the OS needs a way to determine what type of file it is. In order to be a good citizen and play with non-Mac systems, something needs to map a 3 digit extension to a certain type of file when it is copied to the Mac. That is where things get tricky. Lots of different programs will attempt to do this. The Mac OS itself will assign a file type if it copies files off of floppy disks. FTP clients will assign a file type when it downloads a file from a server. Browsers will also do this when they download files. And to make it even worse, these assignments can be changed in the configuration of each application. So as you can see, there is great opportunity for files with the same extension to get different file type assignments depending on how they got there and how each user's system is configured.
Most applications look at this file type data to decide what kind of file each is...not the file name or extension. Therefore, when someone says they have no problem with files of type .yyy or type .zzz, it really doesn't mean much. What is important is what type of file the application that is trying to use it thinks it is.
This is where these other tools such as ResEdit, ChangeCreator, or FileXaminor come in. They let you force a file to have a certain creator and a certain type. You can also use a small AppleScript from Lennart's site.
So, in iMovie, if your .JPG or .MP3 or whatever files are grayed out when trying to import them, then it means they currently are assigned a file type that is not recognized by iMovie. Forget about the name or the extension. You need to use one of those tools to force them to have a creator and type that iMovie will recognize.
Thanks to FunWithMacX for the extended answer.