This is one of those strange situations with Cydia.
Once you purchase a package, Cydia will allow you to install it on any device you choose, EVEN IF the compatibility with your device and version of IOS is not confirmed.
So, in this case, because I (and Rushh) already own this product, Cydia will let us install it on iPad3 5.1.1.
If however you don't own the product and Cydia does not yet have official confirmation from the Author of a tweak that it works, then it will not let you purchase it. This is to prevent the major admin headache of folks purchasing stuff which may not work and then seeking a refund.
The best thing you can do in this situation is get in touch with the Author using the contact details in Cydia and ask him to update the relevant package information so that it will install.
Hope that clarifies.