You buy Photoshop Extended - from the same places that sell Photoshop.
It is is extended you will see 3D in the upper menu between Filter and View.
Extended is serial number dependant, so if you bought extended and it is not contact customer service. They can give you another serial number to activate it.
It is part of CS6cloud download so I expected the PS6 programme to be the extended version as advertised.
When I look again I have 12 programmes shown in Adobe Application manager but on the "lesrn more" page for CS^5 cloud 15 programmmes are listed.
Is there more than one version of the Cloud ?