Thanks. I've reported this to the McMMO devs.

 

@Torzon you must hold shift when using an off hand item. This is the response from the developer.

 

Holding shift while right-clicking will activate the mcmmo ability, but it will also place anything held in the offhand. Ex. Holding a pickaxe in hand and dirt in offhand activates the mcmmo skill superbreaker but also places a block of dirt.

Note: Mcmmo skills functioned properly. Tested with pickaxe, shovel, and axe.

 

Thanks will report.