Nothing right away. Eventually you get into the black widow's lair and use the card to enter her trove of stolen art.
Later in the game you'll go to the "Black Widow's" lair. That's where you'll use the keycard :)
When you get past the Black Widow's guards, then you will use it to enter her layer. Have fun! :-)
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.
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.
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.
The Black Widow is never in the museum (at least not as the Black Widow).
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!
skrow you
The Black Widow (veuve noire) is posing as the police inspector, whose house is in the Country Side. It is on the far right of Counterfeit Island.
the black widow
in the inspectors cottage
beat the black widow