The red magnetic card is a card key used to open certain electronic doors with red locks. It allows to access areas that have been restricted by FunFrock, and because of that, it is very difficult to obtain one.
Twinsen can get a fake magnetic card if he talks with the rabibunny that is held prisoner for traffic of the fake cards in FunFrock's headquarters in Proxima Island. When he speaks to him through a little window outside the fortress, the rabibunny tells him to go see his cousin in Proxima Island, who can make a fake card for him if he says the secret word "Amos" to him.
"Red Magnetic Card. It will allow you to open the doors with a red lock."