@sandy123.. Take a look at this useful tutorial about purchasing add ons made by @srlawren . I highly recommend you preload enough money for the add ons before you need it.. that way in case payment doesn't go through that day, you can go pay by an alternate method ahead of time before you need the add on. Give yourself enough cushion because we all know moderators' response can be slow.
On the day you want the add ons.. I assume you want the US Roaming add ons because that's the only one that is time sensitive. Purchase the add ons and you should see it on the front page under My Data & Add-Ons.
Make sure your Roaming is set on, also network selection to Auto (not locked to Public). Power off the phone when you cross the border (bordering the plane) and power on when you land or cross the border.
