@irvinw Is the money in "available funds" or is the add-on showing up in "my add-ons" ?
Please check this guide for purchasing add-ons, commonly people think they purchased it, but did not actually purchase it as it is a two step process.
US Roaming Add-ons are enabled a soon as you buy them and automatically. Are you sure you completed the 2 step proces? With the US add-ons, you need to connect to the roaming partner networks (AT&T, T-Mobile) in order to use the service. The Public Mobile network is only available in Canada. All roaming usage involves being able to connect to a roaming partner's network. If data is purchased, roaming data needs to be enabled. Did you enable roaming on your phone? You need to make sure your phone has "ROAMING" settings turned to on, on your phone.
In my experience when you click enable roaming on your phone, the phone gives you a warning. "ARE YOU ABSOLUTELY SURE YOU WANT TO DO THIS YOU MAY INCUR MASSIVE CHARGES." Pretty sure all phones have roaming disabled by default for this reason.