Open Devices > Device Setup. Click to + button, and Add a Generic Remote Device. Set your Axiom as an Input, and click Apply. Enable Learn. Select Fader 1 in the upper table, and press the button (for example Play), you want to assign. Then in the bottom table in the Fader 1 line, set Device = Transport; Category = Device; Action = Start.
This is, how it works in general.
For the M-Audio Axiom 49, there could be this settings already done. If you find it, click in the Device Setup window to the Import button. Locate the XML file, and import it.