Thanks for the info on the darts!
Im actually inside the security gate, as I had a keycard but once inside i cant work out where to go, there is another door up to the right but it doesnt give me option to open it?
once your inside the gate don't look for a door you have to go down the steps infront of you and follow the path which will bring you inside the prison tip for you and anybody else if you playing to get achievements you will need the drop ship card.
once you have realised captain marshall(he's the prisoner) don't run off stay very close to him because when you've killed all the guards he runs to get his weapon out of a crate the drop ship card is in that crate make sure you have all cards in the prison before you leave because you cannot return there