If the credit has been applied to your account then it would be showing towards the top of your My Account > My Membership / My Payments whilst logged on here.
Here's an example on my account in the screen shot below (though my credit is not for £9.99 for a Day Sports Membership).
If the £9.99 credit is showing on your account then you need to purchase a Day Sports Membership on your account where the money will be taken from the credit rather than your bank card.
If there is no sign of the £9.99 credit (assuming you only have one NOW account ?).
Then jump over to NOW live chat for further assistance & help.
To reach live chat click the green chat online button from this link page below.