Airport Extreme not working with new modem
The Amber light could mean many things.  

Steps I would take:

Install new firmware http://support.apple.com/kb/DL1481?viewlocale=en_US&amp;locale=en_US

Second: 

Factory default reset

Does your modem have a router built into it?  If so, you need to put that either into bridge mode or router behind router mode.