User blog comment:Kogath/Monster Hunter Generations Discussion/@comment-11537809-20160616002513

Demo is available now (at least in the UK)

http://www.nintendo.co.uk/Games/Nintendo-3DS/Monster-Hunter-Generations-1090846.html

On the side, there'll be a link that says "Free Download", this will lead to you signing into your My Nintendo account which should lead to the download on your 3ds. Not sure if it is directly available on the eShop yet.