can't use b-frames

[PeteSapai]

I wanted to use b-frames with xvid, but couldnt, cuz the checkboxes were all greyed out. take a look at this pic:
http://petesapai.tripod.com/nobframe.jpg
I've searched the forum, but it looks like I'm the only one with this problem. Can anyone help me out?

[Gaia]

Get a new build. You're using some very ancient build from last year.

[PeteSapai]

hmm, I had Koepi's stable version, apperantly I needed the beta version. well, thnx for your help :)