You need to contact your SE and request this. If you are on 7.1, however, you do not need an explicit cluster license as you can create/join on a whim.
FYI only , i am using / testing IVE 7.0 and cluster licencing is also not required from this release.
@michael.saw: you are welcome; glad it worked.
@IanK: sorry, thank you for the clarification that this was introduced in 7.0. I could not remember where it was and went with the OS level I knew for certain contained this change.