trying to install a nexus hub and can't seem to get the lock ring to turn and lock. It is like I can't push it down enough to slip under the tabs that is locks under.

Any tips to doing this?

I`d say double check everything- all parts on in the correct orientation (if you don`t still have the manual, it`s available online from Shimano and Sheldon Brown) and correct order, everything reasonably clean, colored dots lined up like they`re supposed to be. I imagine that`s all obvious to you, but mine has never given me any trouble and doesn`t need to be forced at all. Then again, all models might not have the same shifting mechanism. Is it an eight speed? Have you had that particular hub assembled and working before, or are you putting a new one together for the first time?