The Films on Demand collection shouldn't be asking for a login if you're trying to use it while on campus, so something might be going on with it.  Could also be an issue with a link you're using. Try this link to go directly to Films on Demand and then you can search for the film you need to watch.

http://library.sewanee.edu/go.php?c=4142172

If it doesn't work, if it still asks for a login, let us know and we'll go from there.