at the top of the arrange window there's a rectangular button with a "+" on it. If you click it, you can make a new track. Choose "Software Instrument". At the left of the arrange window you'll see 2 channel strips (you may have to click on the "Inspector" button at the upper left). In the left channel strip, click-hold on the blank grey button below "I/O". Now you can select a software instrument, for instance Ultrabeat. Choose the little triangle next to Ultrabeat, and select "stereo".
Hope this helps!
Kind regards,
Jaap