If you are referring to the keycard in Poptropica Counterfeit Island, you use it to unlock a door in the secret lair, which can be found in the countryside. It is locked so you have to find a key.
You can use it to get the black widow
get the orange keycard in the light bule door.
Grab the keycard he dropped and go to the museum. the assisatant currator has a gift for you...
It opens the door to the treasure room of the Black Widow, who you will encounter soon enough. Go back to the museum and you will get a message from Early Poptropica.
Once you have gotten the keycard after the scooter chase you go to the art museum on early poptropica island and this lady will give you a key that goes to the cottage on the countryside.
You can get it from the person in the case in the Wharf that says "dont open until 2000" give him the coffee from Dr.Romero's coffee maker in the bunker and he will give it to you :)
If your talking about the Keycard for the subway, you get it from the Construction Workers lunchbox at the construction site. But if your talking about the Keycard for the crane at the Warf, you get that from the man in the bunker at the Warf if you get him a cup of coffe.
To get the VIP keycard in fallout new Vegas find the H and H tool company building then up in the office on the top floor next to the terminal there is the keycard.
Later in the game you'll go to the "Black Widow's" lair. That's where you'll use the keycard :)
fatr
To enter the SEBEC building in Persona, you need to have the keycard obtained by progressing through the SEBEC route. Once you have the keycard, simply approach the building entrance in the SEBEC headquarters area and use the keycard to unlock the door. This will allow you to enter the SEBEC building and continue your mission.
on a table in the garage