To use USPS real time rates you need:
1) Weight specified for every product
2) register with http://www.uspswebtools.com/
3) in "General settings" -> "Shipping options" enter USPS server name (production.shippingapis.com) and USPS username
NOTE: after your USPS account is ready you will have to contact USPS web tools support and ask to switch you to production server because your shopping cart has being already tested with USPS.
4) in "General settings" -> "Shipping options" enable "Enable real-time shipping calculation" option
5) in "Shipping methods" menu enable all the USPS methods you want to use and disable everything else.
6) in "Shipping methods" use "Test Realtime Shipping Rates Calculation" link to check if rates are working.