I think you missed step 273 on page 133 of book 1.
In this step you align the front and back wheel and move a gear wheel and bush so the front steering mechanism is connected to the rest.
To fix this you need to disassemble part of the front
After taken of the fenders
Follow steps 471 till 484 in reverse.
This removes the panel above the gear so you should ...
The electrial schematic for the NXT is available in the LEGO "NXT Hardware Developer Kit". This can be found on the official LEGO MINDSTORMS download page near the bottom of the page in the Advanced Users – Developer Kits section.