The nUI Core section, where you downloaded "nUI Release" from is not the same as nUI+ -- nUI Release does not contain the 15, 20, 25 or 40 man raid frames. They are only available in the nUI+ download.
To gain access to that download area, you need to go to
http://www.nUIaddon.com/plus.html and subscribe to nUI's newsletter. That will give you instructions on how to get the nUI+ version which will then give you the 15, 20, 25 and 40 man raid frames.
Sorry for the trouble, it's simply a consequence of Blizzard's new policy on add-on development and my need to have a way to directly and reliably communicate with my users.