You will need an external drive attached and formatted as Mac OS Extended (journaled).
Open iMovie and make sure that your project is working.
In iMovie, look at the Project Source List (the list of projects on the left). You should see an icon for your external drive. Drag and drop the icon for your project onto the icon for the external drive while holding down the Command key.
Next, select the project in the project source list (listed under the external drive icon).
Click File/Consolidate Media. This will move the Event file(s) and if necessary, copies of the music tracks and photos to the external drive while preserving the links in the project.
Now, close iMovie and eject your external drive.
Hook it up to your other computer
Open iMovie. You should see the Project and Event in iMovie. You can leave it on the external and edit it there.
But if you prefer, you can drag and drop the Project icon while holding the command key, and drop it on the icon of your new computer's HD. Then do File/Consolidate Media to bring over the Event files etc.
Regards, Zyfert