The Plebeian Tribunes were elected by a separate body of the Roman Government known as the Concilium Plebis, or the Council of Plebeians. Made up of "Tribes", much like the Comitata Tributa and Centuriata, only Plebieans could vote in it.
To vote in this thread, your character MUST be a Pleb. Non pleb votes WILL be discounted. Keep in mind that votes added by popular favor will make up the overwhelming majority.