If it's a "walking stand" you just come out of the jetbridge into the arrivals concourse. Follow it until about halfway and there is a door to the left that you need to swipe your ID through. Then take the lift down. Walk straight out, around the corner, through the double doors and you are back in JH.
If it's a remote stand, the despatcher should arrange crew transport for you, which will drop you off right outside the double doors.
By rights, you should never have been put in that position in the first place. No one should expect you to know where to go so early on. The reason the ground staff didn't know what to do, is because crew are not allowed to go through passenger passport control and customs, unless you are commuting or deadheading.
If you're joining a crew and it's a "walking stand", just go through security in JH and follow the pier out past the briefing rooms, through the double doors (swipe your ID) and into the departures concourse. Then just procede to your gate. The agents should let you onto the aircraft.
If it's a remote stand, ops will arrange for transport for you and will tell you what bus bay to go to. Go through security and take the lift down to the buses, easy.