Welcome to the AdWords community :-)
You won't actually see it on your account immediately after redeeming the code (I've recently redeemed a code on a new account myself and there was no notification of it) However, the good news is, as soon as the account had reached the spend criteria of £25.00, I received an email saying that the free credit had been activated on the account, and when i checked my billing tab I could see that it had been credited too.
I'm adding some promo code information, as there is specific criteria to using codes, such as one per customer, new account, time sensitivity etc. Please have a look as I'm sure you will find them useful-
I hope this helps