Is the breaker on and the light bulb good? Is the makeup in the lightbox correct? If you put the incoming black wire to the light bulb, does it light up? If all those are good, the best guess is that either the black or the white from the lightbox is not connected to a "common" terminal, but one of the traveler terminals. It is always possible that one of the switches is bad as well - even brand new switches can be defective.
If you have a voltmeter, preferably a non-contact tester, you can troubleshoot it as well. Using the wire colors in the diagram, the white wire at the switch should be hot all the time. One traveler or the other at that same switch should be hot, changing as the switch is flipped. If all that works, and the travelers at the other switch go hot or cold as the first switch is flipped, the black wire at the other switch should be hot or not, as that switch is flipped. Checking these should tell you where the problem is.