if i do that it says :
Please login with a profile that have a valid membership and my membership is valid
You'd be best contacting Microsoft and see if there is any issues with your gold membership, Console or anything else related to the profile that can be causing this problem. Rednecksniper above posted how to contact them.
Everyone can log in with a guest,
Not if the first profile does NOT have Xbox Live.
Your best bet is to contact Customer Support for Xbox Live and then Activision.
Starting with Xbox Live first to clear up any account issues.
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Black Ops, Modern Warfare 3, & Black Ops II