123D Make is free and available to download for Macs at http://www.123dapp.com/make. There are also plans in the works to release an online version of the software which will run on PCs as well as Macs, hopefully within the next few months.
You can now buy a pre-cut version of this kit at the Instructables Store! Woohoo!
Step 1: Start up 123D Make
In the top left, you'll see a tab labeled "Start shape". This is where you choose the object that you'd like to slice. You can either select a shape from the small library of default models, of you can import your own STL or OBJ file. If you've got something other than a T-Rex that you'd prefer to make, then go ahead and do that.
You can download the T-Rex model here and then open it by clicking on Import in 123D Make.