ReSharper doesn't detect implicit usages. You can use [UsedImplicitly] to tell it that your type member is used implicitly, and then it should stop complaining.
In order to use UsedImplicitlyAttribute in your code, you should either include reference to JetBrains.Annotations.dll or include some copy-pasted source code in your project, see http://www.jetbrains.com/resharper/webhelp/Code_Analysis__Annotations_in_Source_Code.html for details.
You should add [UsedImplicitly] on each enum value.