If you are using 2.53 Beta, select the edges you wish to cut. Press K and LMB together, the cursor will change to a knife icon. Drag over the edges you want to cut and release the mouse button.

Although there are options in the tool shelf for exact, midpoints etc., they don't work. You need to select F6 which will bring the panel up in the 3D window and select the options from there.

If you're using 2.49 its just a matter of hitting the K key.