Once the AT-STs appear in the Battle of Hoth, press Left on the D-Pad and the Right 'C' button simultaneously. Immediately after this, press Up on the D-Pad. Now use the Right 'C' button to scroll through the camera views 'til you become an AT-ST. You can use the D-Pad to move this awesome machine of doom and press Up on the D-Pad to fire your deadly blasters.