Click on Transport > Project Synchronization Setup... > Machine Control Input > MMC Slave Active and then select your Keylab model in MMC Input
Keylab transport buttons should work fine but Loop button. Anyway, I don't like the way that Rewind and Forward buttons work at all. When you press and release these buttons then you will have to press Stop. Cubase doesn't work so when you click on Rewind and Forward. The right way would be those buttons worked only while they were pressed. I think this behaviour or mode is called Control Duration in the Keylab Manual but you cannot modify the MMC buttons mode. So I'm trying to modify it through Cubase unsuccessfully at the moment.
Any idea?
The other way would be those transport buttons worked in Control mode and they sent CCs just like Loop or switch buttons then you could assign them a different mode, for example Control Duration. In my opinion that isn't the right way at least for Rewind and Forward transport buttons.