http://www.youtube.com/watch?v=YsVLJGKKunE
Go to the FX mixer. Click on whatever signal you want to control the sidechain with, eg, the kick. Then on the right pane of the mixer click on "add effect" and choose "Peak controller", prolly right at the end of the list.
Now go to the channel you want controlled by the sidechain, eg bass, right click and "link to controller". Choose "user controller" on the window that comes up, and choose the controller from the drop down list (you prolly got just the one you added). Click OK.
Now, with default settings, the channel on the business end of the sidechain (eg the bass) the level will be all the way down. On the "Peak Controller" (PC from now on) the knob "base" controls what the base level is. "AMT" is positive (to the right) or negative (to the left), this controls how much the level will go down (when it's negative) or up (otherwise) when the sidechain kicks in. "Decay" is self explanatory I suppose.