ok, simple:

Please read and save it somewhere for anyone havent redeem

to redeem the free credit: register your phone via http://us.htc.com/googleplay/

Wait for 2-3 week until you get the email with the code
As you havent register the google wallet, here is what I did:

1) go into google play, login ur gmail or ur acc that you register under ur phone
2) click redeem at bottom middle of the page...use PC better
3) you will brought to google wallet page
4) select US country  n payment method later
5) find a US postcode(any also can) and activate/register
6) connect the US VPN (http://www.getusvpn.com/ as provided by @coldfission)
7) then close the google sites
8) open the play store website again and click on redeem at the bottom of the page again
9) put in the code and volla
10) you can buy without VPN, disconnect it