You'd normally create an image (or vector graphic) of your signature in some way and use it as the source for the signature appearance. Acrobat doesn't prompt you to hand-draw a signature and use it for an appearance.
To find the file, you can open launch the "Digital ID and Trusted Certificates Setting" dialog, locate the digital ID file in the list (Digital IDs > Digital ID Files), click "Certificate Details", Details tab, Certificate Data > Private Key <see details>
and it will show where the .pfx file is on your system.
I don't recall how to do the first step in Acrobat 10, but in 11 they changed it to: Edit > Preferences > Signatures > Identities & Trusted Certificates
and in Reader 10 it's under: Edit > Protection > Security Settings