You need 3 people to open the serum lab and it needs to be on the bigger map with 8 tiles.
The serum lab is always located in the center tile northeast, southeast, northwest or southwest if you're playing on normal mode.
The 2 connecting terminals are always in the center tile as well. To find them trace the wires from the serum lab back to them.
Have 1 person at the lab terminal and the other 2 at the connecting terminals press the button at the same time.
Once it's opened there is one serum available, and another that spawns after 5 mins. After these 2 are used there will be no more serum spawns during the game. You can use the serum on anyone in the game and it will count.