Though MP4 is a native supported video format by iMovie, you just can’t import some of the video clips in MP4 format into iMovie on Mountain Lion. According to Apple Support, iMovie can import MP4 files with H.264 codec. Some MP4 file (like MPEG-4 files ripped from DVD by Handbrake) may contain number of audio, video, and even subtitle streams that won’t be supported by iMovie on Leopard, Snow Leopard, Lion and Mountain Lion.
To solve "Can't add MP4 to iMovie" problem, there is no other option but to convert MP4 to iMovie supported video formats. The following is an easy step-by-step guide to convert and import MP4 to iMovie for edit.
Preparation: Download Aiseesoft iMovie Converter for Mac. Install and Run it
Step 1: Load MP4 file
Press “Add Video” button to import your MP4 file. Or you can also drag your MP4 file directly to the main interface of Bigasoft iMovie Converter for Mac.
Step 2: Set output format to iMovie Video MPEG-4
Click the drop-down button on the right side of the “Profile” button to select iMovie Video MPEG-4 output format.
Step 3: Convert MP4 to iMovie supported MPEG-4 format
Click “Convert” button to begin the MP4 to iMovie conversion on Mountain Lion.
Step 4: Import MP4 into iMovie on Mountain Lion
Start iMovie, and choose File > Import > Movies…. Then navigate to the folder where your converted MP4 file is located and select (highlight) the converted MP4 file. Create a new event and click “Import” button.
file is located and select (highlight) the converted MP4 file. Create a new event and click “Import” button.