The Bmajor chord is to go with tunes in Eminor, which often need a Bmajor, though not so much in English or Irish trad tunes, which tend more to use Em and D for harmonies in Eminor. But the melodeon wasn't just designed for those runes: Euro tunes, and popular minor key tunes, are more likely to use a Bmajor. Where you really feel you need a Bminor chord, just play the B bass note with the left hand and bother with the chord button. That should do the trick.