Installing linux Mint 17.1 on Acer s5 "No root file system defined"

I have booted Linux Mint from USB stick. Everything works fine until I'm trying to install it. So I launch Install Linux Mint choose my language ⇒ set up WiFi connection ⇒ Installation type shows up (already different from online tutorials).

What do I see:

If I click "install now": No root file system is defined. Please correct this from the partitioning menu

if I click "+":

How to solve this issue?

I'm using Acer s5 laptop.