I'm using an Apogee GIO to control MainStage, but since the GIO is just sending MIDI controller data, the same thing should work from your keyboard. The way to set it up is to go to assignments and mappings. Look down the list of Assignments and Mappings to find Next Patch, Previous Patch, Next Set, Previous Set, etc. If you don't find these parameters, you can simply create them from "New Assignment" under the little gear icon next to "Assign & Map."
Once you've created the mapping, you need to create an assignment. Hover the mouse cursor just to the left of where it says "No Assignment," and you'll get a disclosure triangle. Click that, and then you'll get a "Hardware Input" dialog box. Choose the device you want to use as a controller (your Keylab), and then choose which channel you want to receive from. For "type," it will depend on what the Keylab is sending; on my GIO I use "Press/Release," but if that doesn't work, just experiment (you almost certainly won't want to use "Absolute," which is for continuous controllers like mod wheels). "Number" is the controller number. You'll want to scroll to the bottom of the list and choose MMC Rewind, MMC Fast Forward, etc. Presumably you don't want to send MIDI through. You can give the input a name, which makes it easier to keep track of when you look at your hardware assignments six months from now and are trying to figure out what does what.
