I believe that was only good during a specific time period. When Elite was introduced, it was a yearly paid service. Then, when BO2 was about to be released, Elite became a free service; Elite is accessible to both MW3 and BO2.
BO2 has it's own Double XP promo which is included with their Season Pass.
So, with regards to MW3's Double XP per joining Elite - it's already over.