Hi,
You can't, yet. The reason for this is that there are quite a few unresolved issues on zooming in - what happens to anchors, etc. Also the tk2dCamera's origin is on the bottom left so scaling that will zoom from the bottom left rather than the center, which will be annoying. If a normal camera wont work for you, drop me an email at support@unikronsoftware and I'll give you a patch with a scale number in there, just as long as you're aware the anchors probably won't work properly there...