There are some tricks to doing the bearings on the rear axle.
Generally its suggested you don't press the bearings off the axle - potential for bending the axle or flange is high. I learnt this the hard way!
Better to use a thin cutting wheel on an angle grinder and cut the retainer ring off, and I did the same to the bearing actually. Its easy to mark the axle doing this, so be careful.
Then press the bearing on to your axle, followed by the retainer ring. Be sure to have put the 4 holed plate that holds the axle/bearing into your housing on first though - again something I learned the hard way and meant I cut up brand new bearings :P
That oil seal lives inside your diff housing. Its the outer seal on your axle tube. Flick the old one out with a screw driver or something and carefully push the new one in, makign sure it seats properly and is flush etc.