first, you might need to ream out the axle holes in the 3D printed cart chassis, they tend to fill with strings when using the makerbot. Fill the axle hole in one of the wheels with hot glue and push in the crimped end of the 5" brass tube. hold it in place while the glue cools. Snap the wooden floor down into the top of the cart. It may need a little trimming to fit properly. Then put the axle on the cart and repeat for the other wheel.