Plus is more about clarity of communication than fidelity of translation.
Unfortunately, there aren't any perfect solutions to this problem. 'X hasn't enough MP' is vague, 'X hasn't enough MP to perform its attack' is more specific, but prolix, and 'X has insufficient MP' just doesn't appeal to me, perhaps because the negation is compounded into a word, rather than made explicit.
Still, 'X hasn't enough MP to perform its attack' may have to do.