Firegpg asks private key

I have created a key for me in my laptop and upload it to pgp.mit.edu..In my school computer i installed firegpg and imported the public key back from pgp.mit.edu when i tried to sign my email messages with my public key it asks me private key..My question is how could i add private key information for my imported public key.. should i have to create a new key with same password to sign my email messages in school?