To curve the bowling ball, release the trigger button (B) and tilt (or curve) the Wii Remote to the left or right. The bowling ball will go in that same direction that you tilted (or curved) the Wii Remote. It is best to do this when you are on the OPPOSITE side of the direction that you are trying to curve the bowling ball, or it will go into the gutter.

NOTE: It does not matter if you tilt or curve the Wii Remote. It has the same effect either way. Also, you must curve/tilt the Wii Remote at THE SAME TIME that you release the trigger button (B).