Note: I'm giving this info based on the MBox 2 Mini, which I used to own, and the Studio Pro 3, which I looked up online.
You will need two 1/8" male mono to 1/4" male mono cables. You should be able to get these at Radio Shack, Guitar Center, or similar stores.
If these exact cables aren't available, you can get 1/8" male mono to 1/8" male mono cables and 1/8" female mono to 1/4" male mono adapters. Then put one of the 1/8" female mono to 1/4" male mono adapters on each cable.
You would then plug the 1/4" end of each cable into the Left and Right Monitor Out jacks on the MBox 2 Mini, and the 1/8" end of each cable into the left and right Studio Pro 3 speakers.
Good luck!