I don't know how you've set it up, but add the folder containing all your movies to the XBMC videos section, on the settings screen for that folder you should have an option to 'Set Content', set it to themoviedb click ok to get back to the video's section, then right click on the folder you just added and select scan.
One thing you might have to take into account is some of the options when you add the folder for example 'recursive scanning' which means that it will delve into all the sub directories in that folder looking for your movies.
Hope this helps!