Dialing *100# will give you your current balance as a flash SMS.
As for GPRS/3G access, you need to make at least one phone call using the SIM first. After that, access will be activated usually in a 1-2 hour timeframe (this somewhat strange procedure was introduced to work around a bug in E-Plus' prepaid billing platform that allowed people to use GPRS without being charged with SIMs that weren't fully activated yet
).
For internet, use the appropriate APN: internet.eplus.de -- AFAIK user id and password don't matter, but eplus / internet is what is documented everywhere. With current day equipment, that should more or less be all you need as far as settings go. How many times and in how many cryptic menues you may have to enter or reference that setting obviously depends on your hardware of choice and
should be documented in the manual ...
Regards,
Chris.
[Edit]
P.S.: To top up, it's probably easiest to buy codes and enter them from your mobile. As has been mentioned, regular E-Plus "Free and Easy" codes will work with the medionmobile card as well -- at least currently. This is not officially documented, and may, for all I know, be subject to change without prior notice. For top-up, you can either call 1155 and select menu item 4 (IIRC) or enter the code using *104*CODE# (again IIRC).