How can I connect the Tamiya double gearbox to Arduino Uno?
Here's what I've found about both the motor shield and the motors:


Motors are 3-6V and will use a lot of amperage. This may be a problem with a mobile robot. You'll have to figure out a power source that can provide 4+ amps... on battery power that's a lot. Personally, I would go with a continuous rotation servo for a compact, light robot.
Motor sheild can provide 2x 2A channels at 5-12 V (Source) using the two + and - terminals on the bottom left side of the shield. You must connect external power, because USB can't provide enough power. I can't find if the motor shield will limit current if it goes over, but if you do use the gearbox, I'd recommend another route for control that's 1/6rd the price.


This chip was recommended by Pololu on the product page.



Time to whip out your soldering iron! Really, soldering is a great investment and you'll get to a point soon where you'll need one; there will be no other alternative. Luck for you, you only have to solder 16 joints. Headers are included.

Diagrams from Pololu's site:


  
  
  
  
  
  *(Click to enlarge ^^^)**


*I added a little text to this image.