Having trouble rooting, someone please help

[Ruuukee]

I am currently attempting to root my GT-I9300 using the instructions on Install CM for i9300 - CyanogenMod. However, at the part where it says to open a terminal and type "heimdall flash --recovery recovery.img --no-reboot" I don't know what to do, i've entered it into command prompt and run but neither of them work, I've followed the instructions to the letter and I have no idea what to do, if I can't get an answer as to where to go from here can someone please explain how to reverse what I've done? any help would be much appreciated, thanks