When you get past the Black Widow's guards, then you will use it to enter her layer. Have fun! :-)
Later in the game you'll go to the "Black Widow's" lair. That's where you'll use the keycard :)
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.
if you want keycard then go to poptropicahelp.net and do island guide
you have to jump to the second box and when the manleaves run be-hid him and jump on the couch and you use the keycard open the door
Nothing right away. Eventually you get into the black widow's lair and use the card to enter her trove of stolen art.
you take the keycard, along with your key, and go to the inspector's place in the countryside. In there, the black widow will be waiting for you, so then you go and jump to the top of the platforms and use the keycard to go through and catch her!
It open the Black Widow's secret treasure trove in her lair. You don't use it when you find it. Return to the museum where a package is waiting for you.
Grab the keycard he dropped and go to the museum. the assisatant currator has a gift for you...
You don't. When the thief escapes after the scooter chase, pick up the Keycard and go back to the museum, where you have a package waiting.
You don't catch him; he gets away. Pick up the keycard he dropped and go back to the museum.
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.
When you are in the Black Widow's secret headquarters, and go past the guards, there is a door with a lock requiring the Black Widow card. Press USE on the card and it opens the door to where you fight the Black Widow.