Hi,

Just open a new arrangement in Music Studio, click on the "Synth" button at the bottom of the screen ("Buttonbar" must be enabled in "Window" menu). A list of the provided virtual instruments will open up, including such drum computer as Beatbox, Robota and DrumnBass. Select one of these. An object will be created in one of your selected arrangement track at the cursor position and also the step sequencer interface for the drum computer will open automatically. Here you can program your beat and set all relevant parameters. Done.

The drum object will automatically be a loop object and you can just extend it to cover more bars.

I hope this helps!