The Axiom, Axiom (2nd gen), and Axiom Pro keyboards are setup to send aftertouch (channel pressure) by default, without any additional configuration.  When the keys are pressed hard enough, the aftertouch strip is triggered and aftertouch is sent in the form of MIDI Channel Pressure messages (CC 131).  While these messages are often routed to control parameters such as vibrato, not all hardware and software respond to aftertouch; please refer to the user guide of the hardware/software you are controlling to find out if it supports aftertouch.

The aftertouch strip can be reprogrammed to send different MIDI messages such as modulation or volume.  Pleaes refer to your keyboard's user guide for more information.