Symptom
When you try to open an iMovie project, this message appears: "Unable to open iMovie project. Final Cut Pro (or Express) requires iMovie 3.0 project files."
Solution
For Final Cut Pro 4
Update to Final Cut Pro 4.0.1 or later (4.1-J or later for Japanese versions). Use Software Updates in System Preferences to install the updates. Note: The Pro Apps Support package (ProKit Framework) must be installed first. Once the Pro Apps Support Package is installed, then click Check Now in Software Update again to see the Final Cut Pro update.
For Final Cut Express
Update to Final Cut Express 2.0 or later.