You first have to send some CAT to Hive account@cat.tips, then you can use the CATtip function. You can type ?catbalance in a comment to find out how much CAT is left in your Hive account@cat.tips wallet. Hive account@catnet will reply with the number of CAT you have in Steem-Engine, how much of it is CATnip, and how much you have in your Hive account@cat.tips wallet.

Later there will be other ways to use it, but right now, that's how it works. The tipping function is still in beta testing, so it's very rudimentary.