I purchased card but don't see the credit

Okay, this happens to me alot. Just go into ur account details thing it should say over 30$ (unless this is ur first time buying an itunes card)

Hello, today purchase the itunes card for $15 and no activate. I can do?

Try going back to the store that you bought it from and ask them to activate the card properly. If they won't do so then try contacting iTunes Support (you will probably need to give them images of the front and back of the card, and its receipt) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then iTunes Cards And Codes