Can't Install Ubuntu onto USB for MacBook

Can't Install Ubuntu onto USB for MacBook

I'm trying to put Ubuntu on a USB hard drive so I can run the OS off it. I
am going off the instructions on this site.
https://help.ubuntu.com/community/How%20to%20install%20Ubuntu%20on%20MacBook%20using%20USB%20Stick
Heres what i put into terminal at step 3:
hdiutil convert /Ti's MacBook Pro/Macintosh
HD/ubuntu-13.04-desktop-amd64.iso -format UDRW -o /Ti's MacBook
Pro/Macintosh HD/Macintosh HD/ubuntu-13.04-desktop-amd64.img
And then i tried this:
hdiutil convert /Ti's MacBook Pro/Macintosh
HD/ubuntu-13.04-desktop-amd64.iso -format UDRW -o /Ti's MacBook
Pro/Macintosh HD/Macintosh HD/.img
Which came out with this:
Usage: hdiutil convert -format -o [options] hdiutil convert -help
Deviating from the guide a little, Would it perhaps be posible to just
rename the .iso with an .img"
I also have a disk copy of Ubuntu 13.04, perhaps could i just burn it to a
bootable USB hard drive instead?
Thanks for the help :-)
Ps, Im not very good at terminal .