Is your account currently enabled with FedEx SmartPost?
There are multiple apps that integrate with Shopify so that you are able to offer FedEx Smartpost shipping. They can be found here:
I can't get it either thru Shipstation even though their site says the shipping services you choose to offer will show up for the customer to choose from as well. Really, we should not have to buy an app for the sole purpose of getting one Fedex product - and then it doesn't work.
Check out the FedEx Rates, Labels and Tracking app, using which you can easily offer FedEx SmartPost shipping.
Apart from this, the app also helps in displaying live FedEx rates at checkout, print FedEx labels in bulk and handle the FedEx tracking process as well.
Smartpost requires you to provide HUB ID and Indicia details. Once you put those details, the solution should be able to show you smartpost rates. With some tweak, you can use the Multi Carrier Shipping label app for Smartpost rates.