Did you remove the knock out on the garbage disposal?

Here is a good straight forward install sheet.
http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType=PDF&Name=31-30515.pdf

To eliminate the house plumbing as a possible cause disconnect the drain at the house plumbing and drain it into a pail. Be sure to hold the drain hose firmly and put a towel over the pail so it does not splash all over.
Usually you can see the drain hose move some when the drain pump kicks in. If it does you may also want to try draining into the pail but undo the drain hose at the air gap.

Can you hear the drain pump trying to run when it should.
Make sure that the electrical connector on the drain pump is OK.
With a meter check that the drain pump is getting power (120 volts) when it should.

If all the above are OK I would get it fixed under warranty.
If you start disassembling the unit they could void your warranty.