[imp] How do I enter the passphrase when signing a message?

[Rick Emery]

First, I connected to my server using ssl. Then, through imp -> options, I
successfully created pgp keys. When I compose a message and select "PGP sign
message", then click "Send Message", I get an error that the passphrase was not
provided. Clicking the help icon next to the option (to sign the message)
reveals that I should be prompted for the passphrase (if it hasn't already been
provided) but no prompt appears.

How do I enter the passphrase so that I can sign and/or encrypt the message?

Thanks in advance,
Rick