The user library is hidden in 10.7 and later. To access it, either hold the option key and click the Go menu while in Finder and choose the Library in the drop down or in the same Go menu, select Go To Folder and enter the path ~/Library.
I will say that most often iMovie instability has little to do with the application but rather the files and other content iMovie tries to index and access upon launch. Try renaming temporarily the iMovie Events and iMovie Projects folders in ~/Movies, and then reopening iMovie. If iMovie launches, something in one or both of those folders is interfering with iMovie's normal operation. If iMovie still doesn't launch, you want to consider renaming or otherwise creating a new iPhoto Library, and then relaunching iMovie. If it still crashes, the iMovie plists or the contents of the ~/Library/Caches folder may need to be trashed. If after all of this iMovie is still out of whack, trying another user is advisable. Possibly something in /Library/QuickTime may need to be removed. Lastly, deleting all the support files and the iMovie application itself, then reinstalling iMovie, may be necessary.