You have to enable Physic's on that object.

Use
ph_setgroup (simpleCube,2); // Assign Object to Physics group 2
and
ph_selectgroup (2); // Enable Phyiscs on Group 2

You may also want to add to the bottom of your PhysicCube Action...

while(1)
{
wait (1);
}

To keep the logic loop alive for the cube....

I hope this helps...

Ken