There have been numerous reports of invalid codes for the Metal Gear Online beta included with the Saga Vol. 2 discs. Your code should be 12 characters.
Some people are getting 9 character codes.
If you are unlucky enough to have a bad code, email firstname.lastname@example.org and ask for a new key. Hopefully you'll get a speedy response.
Your other option is to rush to Best Buy or another big box retailer that has Saga DVDs available for $5 (counts as a pre-order for MGS4).