Just ran into this issue, too, and the accepted answer helped me solve it. To elaborate a bit more: When an assembly is added like this:
myAssemblyObject = NET.addAssembly('C:\path\to\myAssembly.dll');
then a handle to a nested enum (i.e., an enum that is a member of a class) can be accessed by using
myEnumHandle = myAssemblyObject.AssemblyHandle.GetType('My.Namespace.MyClass+MyNestedEnum');
Now, a list of the values of the enum can be accessed by
myEnumHandle.GetEnumValues()
and an individual value at index ix by
myEnumHandle.GetEnumValues().Get(ix)
All the handles (to the assembly, the enum, and the enum values) offer a wealth of additional properties and methods that can be explored by using tab completion in the Matlab command window.