There are two things you can do.
One is make a movies folder in your external drive and make an alias of this in the movies folder in your home folder. When you access this through frontrow you select videos>movies>movies to open the external drive folder.
A second method is to make a movies folder in the external hard drive and drag this into itunes. This will populate the movies folder of itunes. This is slightly more efficient when accessing through frontrow: videos>movies. It also has the benefit of giving you an extra screen to play around with in itunes.
I use the itunes method, though both work fine.
I hope this helps