Do you mean added to your NOW account or bank account?
Is it related to Broadband credit or TV credit?
I have never had NOW Broadband credit before with NOW but I believe it can take up to 30 days for the credit to be added to your account.
When I have received TV credit in the past it was on the top of my online account page under TV Membership and My Bills & Payments.
If NOW have advised the credit has already been definitely added to your account then I am assuming you only have the single account and not looking on the secondary account if you have one.
Also if it's Broadband credit go to My Bills & Payments and click on both recent and up & coming payment to see if the credit has already been applied to the bill.
Clicking on the Broadband and Calls bill price should show if any credit was included and was dedicated from your bill payment.