At this time, NAV 2015 is only available to Microsoft Partners. General availability begins on October 1, 2014 for new customers, and on October 6, 2014 for existing customers.
Login to PartnerSource to download Dynamics NAV 2015 from the link below.
NAV 2015 is available in the following localizations: Australia, Austria, Belgium, Canada, Denmark, Finland, France, Germany, Iceland, Italy, Mexico, Netherlands, New Zealand, Norway, Spain, Sweden, Switzerland, United Kingdom, United States, Russia, and W1 (Worldwide edition).
Happy downloading and exploring the new capabilities of Dynamics NAV 2015.