The Cloud is not CS6, the Cloud would be CS7 (actually is, internally) so I'm not sure you can go from CS5 to 7 with a discount
I don't know that you CAN'T either, so try... Adobe contact information - http://helpx.adobe.com/contact.html
The Cloud is definitely available at promotional pricing (for the first 12 months only) to registered customers of CS3 or later.
If the Adobe website is telling you that you do not qualify then check first that you're not an Educational, OEM or Volume Licensing customer. If so, the offer does not apply.
If you have a commercial retail version then contact Adobe to discuss