It is important to remember your gets' and sets'.

You bone most likely doesn't have a name... So lets use a set to give it one.
ent_bonehandle -
Quote:
Modifies:
name (set to the name of the bone resp. parent bone when both name and index are nonzero) .

Code:
ent_bonehandle(my,"waist",5); // Get the bone number from MED in this ex '5'


If this did set the name, than you should now be able to use the name as a constant

Code:
my.BEND_WAIST += 5 * mouse_force.y;
	ent_bonerotate(me, "waist", vector(0,my.BEND_WAIST,0));