The PCMCIA slot on the 2500 series router was only used for code upgrades. You used to be able to buy an IOS upgrade and cisco would ship you on PCMCIA. You would open the router and plug the card in and Hey Presto! your IOS would be upgraded (if you were lucky... If you weren't.. Well...).
Unfortunatly you cannot actually use this slot for running IOS from, or to write a new IOS onto the card. AFAIR, the card wasn't even a "real" pcmcia card, it just used the same connector
