well as the error says the mydql extension is not yet correctly installed? have you tried restarting apache? if so and it still doesnt work then you need to go ask the debain guys whats wrong, this is not a RC issue so i'm afraid you wont get much help with it here
This is the RoundCube forum, you will not get help with Dovecot here. Also please stop duplicating your posts. you wont get help any faster and its very annoying!