The most common reason FedEx SmartPost rates aren't displaying is due to an incorrect "FedEx Smartpost Hub ID."
Keep in mind the "Smartpost Hub ID" is not necessarily just the closest Hub to your address, but is assigned specifically to each FedEx account. You need to check with your FedEx representative in order to confirm your Hub ID. You'll find the full list of these IDs on the FedEx website: http://www.fedex.com/us/smart-post/network.html
FedEx SmartPost also has minimum dimensions. Minimum dimensions of 6 x 4 x 1 in must be entered to see SmartPost rates.