enum My_Enum {
    enum1 = 0,
    enum2  = !0,
};
typedef enum My_Enum My_Enum;

My_Enum myFunction(int arg);

Declare Function dll_myFunction Lib "C:link_to_my_dll.dll" _
        Alias "myFunction" (ByVal arg As Long) As Long

Private Sub Try_Calling()
    Dim myArg As Long
    myArg = 150
    dll_myFunction myArg
End Sub