The full instructions on how to attach the front wheel to you bike can be found here.

To connect the motor cable:

• Run the motor cable between the motor and the fender stay. 
• Align the motor cable arrows and click them together. Then tuck it into the front fork as far as possible
• Take the motor cable cover and the two cable cover bolts. 
• The motor cable only needs to pushed slightly inside the front fork. If you find it difficult to grip the cable, you can use a pair of needle nose pliers to help insert it into the fork.
• Place the cover over the opening and fasten it with the bolts and Allen key 3. Tighten the first bolt a little to hold the cover in place, then insert and tighten the other bolt. 
• Tighten both bolts to make sure the cover is secure
  

A video demonstrating the last two steps can be found below.