Dude,
If you have an approved I485 you should be able to just walk-in to the local INS office during the time frame and days mentioned on the I485 approval and get your passport stamped. You will need have your approval notice, EAD (if you applied for one), AP (If you applied for one), two photographs and your I94. You will be asked to handover these documents and passport, the passport will be returned with the stamp. You will also be asked to sign and put your fingerprint on the form for the plastic card. Thats all!!
Hope this helps.
gcfasttrack