Thanks for sharing that much information. We will surely help you with the project.
You can connect the L293D directly to any of the digital pin of the controller. Go through the datasheet and you can find the sample circuit for connecting the motor. Pull up the pins which you intent to connect the switch in such a way that when you press the key the digital pin should receive logic 0 otherwise logic 1. Thats all about the hardware.
Now coming to the programming part wait till the digital pin goes low (happens when you press the key) and once low activate the pin in which the l293D is connected, add a delay. Now wait till the pin goes low again and if it does deactivate the L293D pin.
Do all these in an infinite loop.
Expecting your doubts!