I have a Wacom Bamboo Tablet and no issues at all, a little tip from me, try to paint the frames in Sai, save them as a PNG File and open them up in Open Toonz and arrange them how you want them, for me that was much easier, because I can't handle to brush in that program even with a working Tablet.
You can check your Tablet Settings, maybe there is something wrong or with the Drivers, otherwise no Idea, sorry. :s
Many people seem to have issues with the Program, thats quite sad, maybe it helps to do it the way I told you until you find a way to make your tablet work :)