When designing a claw to pick up the large foam balls, two key points need to be considered; stability, and weight. The claw does not need to be complex to complete its task, but the more advanced teams can get creative and have very unique designs.
Mounting off of the iCreate is very similar to mounting off any lego chassis; that is, with the exception of the metal. If you look at the iCreate by itself, it is meant to be mounted off by metal (there's assortment of screw wholes there). However, if you feel you have mastered mounting off of it with the metal pieces, and are feeling adventurous, go ahead and mount off with legos.
Before beginning to build a chassis, you need to understand that this must be the sturdiest part of your lego bot. Because you will only build from here, an unstable chassis makes an even more unstable robot, only adding to natural drift problems and other unavoidable problems with motors and programming. The fist step to avoiding this is when securing Motors, make sure they are snug.