You are entitled to use the ME service as a resort guest for no charge. However it isn't exactly automatc as you have to make a reservation. This can be done online or via the telephone when making your reservation or added later.
You qualify for Magical Express, but you don't "automatically get" the service. You must reserve Magical Express ahead of time.
Anyone who is on a reservation for a Disney-operated resort (including DVC) at WDW qualifies for Magical Express -- even someone staying a single, discounted night at a value resort. Magical Express does not require a package, or a particular rate, or a minimum length of stay.