I had the same thing on my 4510. My father in law made a special tool to screw into the old seal to pull it out. We then, as I recall, used a large socket or pipe to push the new seal in. Not too difficult, a cheap part caused alot of leakage and we still do not know why it should have went bad in the first place. When I drained the remainder of the trans. fluild I find what appeared to be the remnants of a copper seal/gasket (?). No problems since but again unsure what that was also.
I now have a very slight leak out of the front wheel drive gear box area. Not alot but messy and annoying. The tractor has less than 1000 hours so I would not think these things should have occurred. Otherwise pretty satisfied for a cab/loader tractor four years old for under $14000.