Far Cry 2

I found that you have to do 2 missions for this "underground" mission to become available (I did 1 mission for each faction). If you then check what missions are open to you (press start) you'll see that an underground mission is available from the priest (?) in Pala - he'll ask you to deliver some passports to a vet and you'll get your meds in return. Go to the yellow marker on your map to complete this mission to get the achievement.

NOTE - the achievement does not pop until you leave the house (in case you worry it isn't going to unlock!). Also the house you have to take them to will be surrounded by about 6 militia members you have to take out before you're allowed to go in - so make sure you're ready for them.

Once you complete your first U.F.L.L. or A.P.R. mission once you pause your game, one of you objective's will be to find someone that can give you medicine. Go back to the church and then the guy in there will give you some travel documents and tell you to deliver them to someone. Look at your map and there will be a yellow marker indicating where you need to go for the medicine. Just go the destination and go inside the house, talk to the guy in the center of the room, he'll tell you to go the the back of the house to receive your medicine. Achievement Unlocked!

For this you simply have to complete your first underground mission. After completing a UFLL or APR mission in the North, you'll be given an underground objective, marked yellow on the map. Travel to the base indicated and you'll be given stamped travel documents. Then, head over to the underground location indicated on the map. There are a few baddies, but they're easy to take out. Once you trade in your documents, you'll receive the medicine. Once you exit the house you'll unlock the achievement.