How do i know what resistor value, that's attached to a BLDC motor, i should use in order to get a torque of 100 lb-ft put on my actuator?
It seems like you're trying to use a BLDC motor as a passive, constant toque load by shorting the leads together.
This is not possible. When shorting the leads of a BLDC motor it behaves variable torque load. The following equations relate motor speed and torque to voltage and current: $$ V = K_e\dot{\theta} $$ $$\tau = K_{\tau}I $$
Substuting, the equation to determine the motor torque when driven as a generator with the leads shorted together is as follows:
$$    \tau = K_{\tau}\frac{K_e \dot{\theta}}{R}$$
As you can see, the motor load torque is directly proportional to the rotor velocity. Assuming your actuator could maintain a controlled 3000rpm, the motor in theory would only produce ~48ft-lb of torque maximum (with the leads all shorted together). Of course, according to the datasheet, the motor is only rated for 10A of current, which means that the motor would be slag long before it got there.
As a side note, 100lb-ft is quite a bit of torque. This motor when driven actively is only rated to produce ~2.4ft-lb. I'm wondering if maybe you made a mistake with your units somewhere.