You aren't actually allowed to stop a car in front of the Embassy entrance (if you are unsure as to why, google "Islamabad Marriott") so our driver pulled up a few blocks past the entrance and we walked back down the hill. Once there, you enter a small anteroom with a metal detector, the purpose of which was a little unclear since it beeped as we both went through but no one seemed to notice. We showed our passports and were ushered into a second security room where we had to turn in all our cameras, recording devices, and go through a second metal detector which the security people seemed to take a little more seriously. You get a tag to reclaim you cameras when you exit, and then go down the hall and out into a courtyard. From the courtyard you follow the path around, go up a ramp and into the waiting room for the visa processing division.
When we arrived the room was packed. Ethiopians, Americans, and lots of kids. It was loud and stuffy and, if you're me, a little claustrophobic. They do have a little gated off play area which would be nice if you kid was a toddler. There is a loudspeaker which announces whose turn it is. When you hear your name you go up the stairs and up to the counter. We waited about 2 hours. Some people were in and out within 30 minutes.
Once you get upstairs you show your passports, sign some documents and answer a few questions ("is this the child you were referred?" "Where is the birth mother?" "Where is the birth father", etc.). Our agency told us these questions are mostly perfunctory and advised us that if we weren't sure we should just say we didn't know. The whole interview took about 3 minutes. Papers are stamped and you are handed a pack of papers which include your child's Ethiopian passport and US Visa, their Ethiopian birth certificate, and the court decision. Don't loose those.
Go back out the way you came in, collect your camera, do not do what we did and try and take a celebratory family photo before you get out of the embassy or a very scary security person with a big gun will demand to review all of your pictures and give you dirty looks, and carry on your way.