Use whatever alpha was indicated on your order (e.g., for 2013 it's 10.4% for the pellets). I don't think the beta matters a whole lot for Beersmith's calculations, but you could put in maybe 5%? I never saw anything about Belma's beta levels.
Actually, I think Belma is covered in the 2013 hops update under add-ons! Try adding that first.
Totally my own opinion, but Belma is the one hop I won't use. It ruined a couple batches for me, even as a pure bittering addition. There's something about the flavor I find really unpleasant... not a good solid bitter like other high-alpha varieties. As a flavor hop, I find it really weak, but the flavor it does add is a weird, overly sweet berry flavor. Anyone in Portland OR who wants an unopened 1 lb bag of 2012 Belma is welcome to it!