You need to use Win32DiskImager on windows for example to write the image to your sdcard, this will however completely overwrite it.
I'm currently using recalbox but i'll TRY to build my own image with attractmode once i've figured everything out... or i'll wait till nitrogen updates his thingy for the pi 3 
EDIT:
Got my image working.
1. Installed raspbian
2. Cloned Attractmode and build it
3. Used the RetroPie setup script
4. 
5. Success. 
Well partly... i have to launch X so i can launch attractmode, for now i can launch snes games in a small window via snes9x.
Any help? I'm a linuxnoob and surprised i actually got this far 
i do have the "attractmode_rpi.img.tar.gz"
i do have the "Win32DiskImager"
when i surch for the attractmode_rpi.img.tar.gz file with Win32DiskImager it doesnt pop up it doesn't think its an image file. who do i fix that?