On the forest level where you must find Sam, Merry and Pippin, run ahead and eventually you will find a pile of rocks in your way. Run back to the gate and then run to the pile of rocks again. They will have disappeared by that time, at which point Frodo will remark about the forest having a mind of its own. From there on, kill the huge spiders with your walking stick, and when you hear a voice say " Hey, Frodo!" or " We're over here!", look around until you find one of the Hobbits.