What could cause my newly installed disposal to not work?
Your new disposal should be connected to a switched GFCI outlet.

Verify that the outlet is not tripped.

Using an outlet tester, 
verify that the switch properly powers the electrical socket.

Many disposals have an integrated breaker. Locate this switch and verify that the disposal itself has not tripped. You could also try plugging the disposal into an extension cord into a known-good outlet and verify that it operates properly.

Verify that the disposal is not jammed. All modern disposals have an allan socket beneath them which manually turn the blades. Make sure this moves freely.

Following all of the previous trouble-shooting tests, replace the disposal.