Since this has been a fairly frequently asked question:
This will be coming on November 9th by end of the day to you via XMB message and email.
You had to have spent at least $100 in the PlayStation Store in the month of October to qualify for this.
Actually this is what was posted on twitter by playstation itself:
Update: U.S. users who purchased $100 of PSN content in October will earn their bonus $20 credit between 11/9 and 11/16 via XMB message!