Custom enum as application setting type in C#?

Sure - just add a serializable enum to your project, select browse and type in the namespace qualified name, e.g. ClassLibrary1.OperatingSystems. Bingo.

"I set the type to my Enum, but in the Value (where Windows, Macintosh should be) only Windows is visible and i'm able to enter any string".

Have you tried entering something other than 'windows' or 'macintosh'? The behavior you see is as close to what you want as you are going to get. I am pretty confident on that.

In any case... good luck.

See it done in 30 seconds here.

Small addition to all previous answers. As for me - it was needed to BUILD solution before my custom enum has been successfully added to the "Custom type" text box.

I see this in VC# Express 2005. The Browse.. "Select a Type" dialog shows only the System and Microsoft namespaces. However if you insert the full name of the type into the Selected Type textbox it should accept it.