You don't need to get any app.
The Boxee Box has built in software that will organize and display your movies and TV Shows in a library format.
It will retrieve the metadata on it's own. This is all assuming that your box is connected to an internet source.
To make it easier for your box to recognize, you should think about putting all your movies in their own folders and throwing in all the extra subtitle files in the movie folder that it corresponds too. As well, the subs should be named the same.
(in the same folder)-Avatar1080p.srt
Polk Audio RM20 5 Pack
Polk Audio PSW125