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.
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.
Later in the game you'll go to the "Black Widow's" lair. That's where you'll use the keycard :)
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
When you get past the Black Widow's guards, then you will use it to enter her layer. Have fun! :-)
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.
You can use to beat the final Boss of Counterfeit Island. Black Widow go to the Inspector house in the east and use a key to open the door. there you can see a painting that has been scrapped in the corner so click it. then you see the scream then the lights will go off and you get captured by Black widow but the real stealer of the scream is innocent. all you have to do is to get close to the thief without falling with your chair. and then when you get close you get free. go to the east of the room you can use the keycard to open the final boss' room in Counterfeit Island. Note: Be careful of the guards
You take it back to counterfeit Island and use it on the door of the house.
After you are in the Widow's lair, you can use it to open the final door to her trove of stolen art.
You use it.
If you have entered it from the tunnel system then you can exit it the same way. if you have entered it from the door you can use the door. if you need to get into the supply room you have to get a job first which means you have to talk to the assistant curator. once you get the job he will give you a key to the supply room. i hope your question was about counterfeit island. also i hope this helped.