Hub size with omni wheels?

Hello friends! I'm making a small 3 wheel drive bot with omni wheels but i just have no clue what hub size for these wheels. Can anyone help me out? http://www.robotshop.com/productinfo.aspx?pc=RB-Nex-08&lang=en-US

Thanks for your time

18mm according to http://www.robotshop.com/content/PDF/layout-14049.pdf

im just confused, how can you mount a hub on that inner circle when there's no screw holes???