Your Account number is your entire mobile number including country code.

If you like you can have your account number and secret code sent to your phone by clicking on Send Secret Code.
OR
To find your Account Number and your Secret Code, start one of the PrizePlay games on your phone, go to MY Account, use the FULL number listed next to the Phone field . Scroll down and select View Secret Code to see your secret code. Enter both of these numbers in the appropriate fields.