How do you do automation on a bus track in sonar?

You can definitely create bus automation. Are you on X2 or higher? If so you even get automation lanes on Busses.
 
You have multiple choices.
 
You can do live automation by making sure the Write Automation button on the bus is enabled (or if you want to automate an effect in the FX bin make sure the Write Enable button in the FX gui is enabled) then start playback and use your mouse to adjust the parameter you want to automate. It will create an Automation Lane for the bus to write the automation to (click the Automation Lane button on the Track Control pane to expand Automation Lanes). You can then edit the automation in the lane.
 
You can use a MIDI controller to do the live changes instead of the mouse. Map the parameter(s) you want to automate to your controller by using the MIDI Learn function or map it the traditional (you'll have to check the manual because I haven't done too much of this and forget the exact steps). Set the bus or FX to Write enable as described above, start playback and fiddle away at your control. The changes will again be written into an automation lane.
 
You can draw automation like you would with a track by expanding the bus automation lanes, click the automation parameter menu dropdown in a lane (add a lane by clicking the + button if you need to) and select the parameter you want to draw automation for.
 
It's basically the same as tracks. If you are on X1 or earlier I'm not sure exactly how it works or if you have as many options but you should at least be able to do live automation like the first two examples.
 
Cheers.