The alias would be made of your external drive Movies folder, and be placed in your Mac's Movies folder.
I use Rivet myself. It allows you to add multiple folders. I have my Mac's movies folder as well as my external movies folder. I think Rivet also allows better organization of how the lists are displayed. (subfolders on the xbox). For me, Rivet also streams to my PS3 so it serves a dual purpose for me.
I'd recommend looking into Rivet, though from my experience both are good programs.
Does Rivet allow streaming of the movie artwork?