Some info here...
If the external HD just contains your iTunes Media folder (where the actual songs/movies are stored) then you don't have any of the library files until you get your iMac back. By default, these live in the iTunes folder in your iMac's music folder, regardless of where the actual media files are located.
But if that's the case I'm not sure how you did this...
Held down the option key while opening iTunes, chose the iTunes library I always use with my iMac
For iTunes to even attempt to open a library, there has to be an ITunes Library.itl file there, which would not be in your ITunes Media folder.
A quick fix would be to drop the whole media folder from the external HD on to the iTunes on the MacBook and let it import everything. Before you do that make sure the option to copy music files to the iTunes Media folder is NOT ticked in Preferences/Advanced.
Thanks for trying to help. I called Apple and after much waiting I was transferred to another level. What we ended up doing was creating a new library on a different external HD which immediately populated with everything from iCloud. I have iTunes Match, so music isn't an issue. And any new movies or TV shows, I will just download into the new Library.
We could not find any itl or xml itunes file in the library I use with my iMac. So why does it work for my iMac.
That is still unanswered.