The NI site says it needs Kontakt 5 but it says the same thing about Scarbee MM Bass but I'm using that with K4.  It installs OK but when I add it to the library nothing happens and it doesn't show up.  Any ideas?

The bad news - NI changed it so it won't run in Kontakt 4

An old version will work in Kontakt 4, but a new version won't so if you have just bought it you need Kontakt 5, likewise if you have an old version and apply the latest updates.

The good (or at least better) news is you only need the free Kontakt 5 player, and it can co-exist with a full version of Kontakt 4.

Kontakt 5 is Windows 7 only, but it works in Vista and XP

That's really sneaky!