The entire thing pivots back and forth. 

Edit:  I should clarify, I was talking about the MT800 above.  There are a fair number of them out out there.

You can also get a kit to make the MT700 have a steerable hitch, but that has cylinders on the lower arms.  I can't say that I've seen that setup before personally, so don't know how many are really out there...  The amount of steering would depend on the space between your tracks.