Where to put enum, inside class or outside?

To an extent it's a matter of personal preference, but my general rule is "If it's class related, put it in the class"

There are reasons for putting in within the class:
1) It reduces the "Pollution" of the namespace by organising the structures in a hierarchical manner.
2) It groups related items together.
3) It lets you control the scope of classes which can use the enum - if they can't access the functions or data which refer to it, then they have no "need to know" of it's existence.

Others will tell you it means more typing when you use it: yes it does. And that can reduce readability to an extent. But I think that it's a good thing, because it ensures that you are using exactly the class / enum you think you are, not a similarly named one from a different source.