I don't know what's going on but when I tighten down the compressor wheel nut or the back-plate bolts (the 4 that attach the compressor to the chra) the compressor wheel seizes up. When they're not tightened down the wheel and shaft spin freely. I don't know what to do.

 

What kind of turbo is it?
Sounds to me like the compressor assy isnt seated correctly before you try to tightened down..

All I can suggest is give it a bit of a wiggle and make sure its pushed back as far as it can be/seated properly before tightening it down..

 

Item #5 is probably installed backwards. One side has a chamfer that is supposed to face the bearings.