I have entered the code into the redeem code section of the marketplace, and I think it told me to log off. However, since last night, when I entered it, nothing has changed. Is there anything I should be doing?

Can you be more specific? What happens when you try to log on?

I can log on, but it doesn't say I am a gold member. I have tried using the code again, but it says it is invalid

Is it a new card? The card may have expired.

I got it with Fable II. It accepted it the first time

Try going to your Account Management and see if you can find any hint that you ever activated the card.

If I try entering the code on Xbox.com, it tells me it has already been used

its working. go play some games!...unless you wanted one of the other perks...like downloading the awesome Banjo Kazooie demo. then id suggest making another account cause youll get 30 days free of real gold.

Its just one of those 48 or 72h cards right?  It shold specify on it that it doesnt provide all of the Gold benefits, but it will allow you to play online.

That Fable Il card was good for 48 hours. Sorry man. That's how they get ya! They give you a free sample to get you hooked, and then they charge you $50 for it! Just like my ex-girlfriend!!

Make a new account for 30 days of XBox Live Gold trial. You can repeat this indefinitely, make new accounts and get free months on each account of XBox Live Gold. It worked for me and it will work for everybody else out there. Only catch you have to change your Gamer Tag monthly.