As you appear to be on Mac OS X 10.7.4 (Lion), your user Library is hidden (as it is on Mountain Lion).
To access your user Library, in the Finder menu bar click on Go then press and hold down the Option key on the keyboard. The Library folder will appear in the drop down list. From there you can navigate to the Preferences folder then locate the iMovie preference file.
As AppleMan mentioned, for iMovie '11 (that's version 9) the file is named "com.apple.iMovieApp.plist". For iMovie '09 (that's version 8), the preference file is named "com.apple.iMovie8.plist". Earlier versions will have a similar name, some without a version number, such as "com.apple.iMovie.plist".