I'm not sure it can be used to extend the existing live gold membership. However, I'm new to live gold and it worked for me.
Steps:
1. Click on 'Go to Deal' link
2. Enter the code M95MN in the upper right corner
3. Fill out the requested form. No need to contain your real data but need a real email address.
4. Answer the question 'are you a Xbox Live member' with no!
5. Check the email and find your 25 digit code (may be in the Junk Mail)
6. Enter the code via Xbox.com > Xbox live gold> Redeem Code
Note: You need to create a Xbox free account if you don't have one.
What's the free game this month?