Dimmer switch doesn't turn light on unless other 3-way switch is also on
Re-check that the correct wire is connected to the single-side of each switch.  For one switch, that's the hot lead from main power.  For the other switch, test the voltage levels on all three leads when the switch is removed.  Two of the leads will alternate between 120V and nothng when you flip the other switch; the third lead is the output line and that's the one you connect to the single-side of the second switch. 

There are lots of diagrams online, and most likely you can get a copy of the one that came with your Lutron switch at their website.