There's a few things that go into this. First, we need to remember that the Mage actually represents the Player within the game. The Mage is the Player and the Player is the Mage. You are the Mage. (See the Codex section for Mage)
Second, you need to recognize that all card text is written as though it is talking directly to the Player. Whenever it tells you to do something, it means you the player/mage.
Now, the directions on Crumble are a little ambiguous. It doesn't actually specify who should gain the mana. But, let's forget about Mage Wars for a moment and imagine a normal conversation. If I am talking to you directly, looking right at you, and say "Pass the salt," would that be a confusing statement since I haven't specified who I'd like to pass it? I don't think so. It is normal in English grammar (I don't know about other languages) that when you're talking directly to someone and give them a direction or make a request of them, you don't have to specify "you" as it is assumed.
So, now when we look back at Crumble, it seems reasonable that when it gives the direction "gain 2 mana" that this is directed at the person being spoken to. You the player/mage.