My opponent might have an Electivire on his bench and therefore HE can get the added attack bonus, but do I get the bonus as well even if I don't have an Electivire on my bench?
This is regarding Magmortar's attack, Twin Bursts.
No, you can't . The card says : if Electivire is on your bench, this attack does 80 more damage. It doesn't say : If an Electivire is in in play, or if an Electivire is on the opponents bench, then too you get the damage