It's likely to be the lever that switches the thing on when you stuff paper in.
As for removing the top, you'll need to look for screw holes, possibly deep ones.
Another angle if attack is to see if there are any snap clips holding body and soul together. I usually use an old fashioned bone handled table knife with a nice, thin, springy, rounded end blade to do probing.
Having played with the odd shredder before, it'll probably be better to resign yourself to a new one.