Well since you already seem brave enough to yank the trackball out (and since you cannot select you really have nothing to lose unless you are under warranty (I that case run don't walk to the nearest store)... Ok (AFTER removing the battery), remove the trackball and under it you will see what looks somewhat like a copper "spoon" this piece presses down on a contact to perform the "enter" function. GENTLY pry it up so that you can see under it. You might see some green corrosion. You can use a knife or small flathead to gently scrap it off (both the bottom side of the "spoon" and the contact below. Blow some compressed air when you are done install the trackball and give it a shot.
Let me know!