The first step after you sign up for Spark Shipping is to ensure connectivity between your store and Spark Shipping. You can test the connection under your site's settings: 

You can also tell if your site is connected by checking the Orders and the Products pages. All your active products should start syncing in Spark Shipping and all orders up to 30 days old should also sync into Spark Shipping. 

You can check out if there are any connection errors in your site log. Go to your dashboard > Scroll Down to find the link to your site log.