When you click the secondary fire the tow cable should be launched. You know the cable is hooked when it changes your view. Try coming up from behind the Walker, on the right side. When its directly on your left hit the tow cable. It'll take several attempts to get used to it, but basically you just go around in circles. Personally I have found it better not to slow my Speeder, just let it go at a normal speed. You'll get the hang of making the circles, eventually just holding the joystick in one position.
1. You go up and down too much. The Speeder automatically hovers so don't worry about touching the ground -- although don't slam it hard!
2. You get too close to the legs at explode by hitting them.
3. You get too far away and the cable is lost. If this happens, you need to make tighter circles. You're actually farther away than it appears at times.
Hope it helps. It was the Walker on Level 4 that first brought me to this site a few years ago. Didn't post then, but read the tips and walk-throughs.