I don't see why a two-year old battery would be giving you trouble unless it has become discharged a couple of times (it sounds from your message like this is the first time you ever haver battery trouble).
I would definitelly have the alternator checked.
To answer your question on the location of your battery:
The battery is underneath the rear seat. The rear seat cushion must be removed by unlatching two small levers (at either side of the rear seat / must stick your hand under the seats' corners). The seat cushion is then pulled up and forward. The battery should be under the right rear seat.
ONce again, I don't think it is the battery. I have a 1999 E-320 and the original battery is still going strong (knock on wood!!!)
Good luck; let us know how it goes.