Old thread but incase you didn't know
middle C is not the lowest c on the guitar.
The guitar transposes down one octave, which means that when you read a note on the stave, the actual pitch sounded is an octave lower.
Therefor when you read middle c on guitar music, you play it usually on the A string at the 3rd fret, this note is actually one octave below middle c.
Middle c on the guitar is located on the B string, first fret.
Sorry if you already know this - a lot of people don't (they do now!)