I just installed Cubase SL last night and had the same issue. What solved it was twofold. First off I had to go into the device setup menu and change from the ASIO driver to the ASIO/Direct X driver. At that point I got audio. Then when I shut the program down and re-started it there were error messages about not finding my soundcard, and no audio would play. I went into the VST connections (I think, from memory) and had to manually select the soundcard and add the channels in the output tab. After that, all was fine.
Hope this helps.
Darryl.....