Problem creating partition, GParted - Mac OSX

Hello

I'm following this guide:
http://docs.kali.org/installation/ka...n-mac-hardware

When I'm supposed to resize the HDD I can't like the guide. I have a yellow/white symbol and the file system is unknown and I can't click "resize", it is greyed out.
The yellow/white warning is indicating this:
Warning:
Unable to detect file system! Possible reasons are:
- The file system is damaged
- The file system is unknown to GParted
- There is no file system available (unformatted)

GParted looks like this for me.
http://i.imgur.com/vxJ1bBd.jpg



Computer:
Using a MacBook Pro, mid 2012, 13", 500GB HDD
OSX Yosemite installed


What I have done:
Inserted the USB-stick with the kali .iso file on and restarted while holding the options-key (ALT) and booted Kali through, Live (amd64).
So now I'm stuck in the GParted step since I cant resize.

I don't use MacBook, but I am thinking that is always better way to shrink partition from OS itself, rather then use gparted from any live installation.
just find partiotion tool for your Mac and shrink partition from there.