So first get the newest version by installing uDig 1.2 from the main download site. Make sure to download version 1.2.
Then install the latest JGrass plugins in uDig. There is a great presentation by Silvia that explains how to do that:
It should then look like the following:
At that point you can follow back the slides.
If you need an introduction on the use of JGrass, follow the presentation until its end.