If I want to hit a Mechadendrite, how can we consider it in terms of TB and Armour?

in the case of a Bionic Limb it has the Character TB + 2, but for a Mechadendrite?

I think to apply the character TB with the Machine Trait (Armour = TB) but I don't know if it's right or not....

I would treat it identically to a bionic limb, with exceptions on a case-by-case basis. For example, a utility Mechadendrite, which is a great heavy slab of hydraulics and heavy machinery, would be dramatically tougher then, say, a precision-milled Medicae Mechadendrite, and so maybe Machine (4) for the Utility, and Machine (1) for the Medicae, to reflect the opposite ends of the spectrum. Nifty little side effects could include accidental poisoning as the Medicae toxins entered the TP's bloodstream, the plasma pistol gun-Mechadendrite's flask exploding, the Utility simply flopping to the floor, effectively hobbling the character, etc.