Go to: http://www.autodesk.com/store/recap-360
Select "PAY-AS-YOU-GO" and then the options for "Annually" and "Basic".
Click "Add to Cart" to be taken to the order confirmation page.
To purchase more cloud credits:
Go to: http://www.autodesk.com/store/configurator-360/cloud-credits
You then have the choice of monthly, quarterly or annual plans. Monthly plan includes 10 cloud credits, quarterly plan includes 30 cloud credits, and the annual plan includes 100 cloud credits.