Got a santa cruz chameleon, which I'm running with HTII and middleburn 26 and 38 toothed chainrings. However, been getting a lot of chainsuck when shifting down, and also the chain falls off to the inside now and then.

http://www.chainreactioncycles.com/Models.aspx?ModelID=17282

Will that solve these problems? I understand I need the BB mount one.

Or do I need/will I be better off with something else?

Chainsuck tends to be worn rings and whilst it probably will work surely better to chain rings as it will cost about the same?
Could try flipping rings and wearing the other side

EDIT: e-mail in profile I have a single ring chain guide BB mounted if you are interested in suggestion below

Brand new chainrings

that will help a fair bit, as will suggestions above, as will running your chain as tight as you can get away with. i think belting a cham round means you lose your chain fairly often. i got sick of it tbh and went single ring with chain guide. worked a treat.

So it will work with the 26 and 38?

You have brand new chain rings and are getting chain suck have you checked the chain for wear?
The one I have has a cog on the bottom guide so would only be suitable for a single ring. Not sure if the crc one works sorry
Seems very odd to have suck on new rings and I would want to know what is causing it before buying something to fix it personally.

New chain as well.

that is rather weird when does it fall off when changing down if so is the limit screw set properly?
When you say chain suck do ou mean the chain is failing to disengage from the ring and is carrying on round and jamming or is it just slipping?