Some router plates are not adjustable and a centering pin will not help. In this case if the plate isn't correctly centered you have to buy a new plate. At the time I purchased one from Pat Warner.I just bought a set of router bushings from Rockler and I'll point out one great benefit of their set. It comes with centering bits that go in the router collette and help you to set up and center the base/bushing. Then you remove the centering bit and insert your router bit an you know it's centered and correct.
None of the el-cheapo sets on Amazon or eBay had these extra two tools so I spent a few extra $$ and got the Rockler set.
