If you purchased Adobe CS5.5 during the CS6 upgrade promotion between March 25th and May 6th, you are now eligible to receive your free upgrade to CS6.
You should have received a one-time use code that allows redemption for your free CS6 upgrade in an Adobe email newsletter sent to qualifying customers on May 15th. If you qualify for the free upgrade and have not received your coupon code, contact Adobe (or check your spam folder).