subtitle help?!

hi i have an xvid video and i have a *.sub subtitle and i can't use it. the subtitle option is unselectable how do i get it to work? thanks so much!

[adicoto]

If the subtitle file is some 3-10 MB large then is a DVD subtitle. Install Vobsub.
If is 20-100 Kb then try to open it with Wordpad for example. See if is a text file something like this:

0:0:10;05-->0:0:20;15
This is a subtile line

or:

{1200}{1300}This is a subtitle line.