I found a simple 555 based oscillator circuit that has controls for adjusting the frequency and pulse width that should be able to operate the speed controller on Electro Schematics. They have both the parts list and circuit board layout to use for making one. You can also buy a ready built unit on Ebay with a display showing the frequency and pulse width on a small screen for $3.50 that uses push buttons instead of rotary knobs.
http://www.electroschematics.com/5834/pulse-generator-with-555/
What pin are you using as a reference for the PWM signal input to the motor controller board? The White Line Neutral wire or the +24v on JP4?