How to use enum within a struct in ANSI C?

Question

Following code has to be used in the main-function, but I don't know how it is used.

struct SomeItem
{
    enum {MOVIE, MUSIC} itemType;
    union {
        struct Movie* movie;
        struct Music* music;
    };
};

this struct is used in a dynamic linked list with previous/item/next pointer, but I don't know how you can set the enum. Or how to initialize it.

I need to know how it would look like in the main-function.

biglist.someitem = ???;
/* declaration I use */
struct Library* biglist;

more code to understand what Im trying to do.

struct Library{
struct SomeItem* someitem;
struct SomeItem* previousItem;
struct SomeItem* nextItem;
};

compiler errors: C2037: left of 'someitem' specifies undefined struct/union 'library' C2065: MOVIE: undeclared identifier

Im still a rookie on ANSI C, so dont shoot me ok ;)