The Hobie Trax wheels have a spring clip that engages with a groove in the end of the axle.
The clip is installed on the twist-knob as part of the wheel assembly. The clip has two parallel lengths of metal that flex to pop around the end of the axle when installing. Perhaps a simple whack with your palm will pop the wheel in place or turn the knob 1/8 turn while pressing the wheel onto the axle.
There is a short pictorial on the Wheeleez site that helps to re-install the spring: http://www.wheeleez.com/repair/twistknobs.php