Does encryption require a certain key?

I know when you sign you have to tell it what key to use but when you encrypt the message do you have to choose the key. Does it use a general encryption method or does it use your key to encrypt the message a certain way?

I think it requires at least one key as I cannot select OK to encrypt without having a key highlighted. I have signature Options set to None, Encryptions Options set to Public-Key and I have the default Append to existing signatures (which I cannot chagne). Even when I change all of the options I have to have a key selected before the OK button ungray’s.

what drives me crazy is; Why can I see the public keys that I loaded on one machine but not on another machine when on both machines I followed the same import process? I see the key’s on both machines in the keyring but not in the sign/encrypt files window.

Was I supposed to install this software as an Admin or somethign?