You can disable confirmation message in the extension's options.
Change the default security/encryption setting in your account settings from "Required" to "Never (do not use encryption)" to let this add-on decide and encrypt only if possible.
For the list of available certificates see menu Tools -> Options -> Advanced -> Certificates -> View Certificates -> People.
Note: this add-on relies on Thunberbird's built in encryption functionality. You have to set it up first before using this add-on, see http://kb.mozillazine.org/Message_security for details.