To be eligible to purchase the Dreamweaver CS6 upgrade and use a CS6 upgrade serial number, you must already own Dreamweaver CS5 or CS5.5 and have a valid serial number for it.
If you do not then you have purchased the wrong product for your needs.
In that case, you must either purchase:
1) Full version of Dreamweaver CS6, or
2) Purchase a Creative Suite, or
3) Join the Creative Cloud
If you do not have a valid CS5.5 serial number to validate your CS6 upgrade serial number then contact Adobe to discuss your options: