You don't say what type it is, but they usually have little micro switches fitted to ensure the power won't come on until the blade section is fitting properly to the waste container part.
If it is the bucket type with the shred part on top just make sure it rests properly on top of the bucket so the microswitch on the bottom of the shred part gets depressed.
If it's a drawer type there may be a small projection on the drawer which pushes against a switch in the body of the shredder.