fdisk is located in /sbin. As a normal user, it's not in your path. When you "su", you gain root priviledge's, but that's it. To run it as you are, you would have to tell the system where fdisk is with the full path... "/sbin/fdisk".
Or, you can "su -". This is pretty much the same as logging in as root, which run root's start-up commands, which sets the path list the system looks through when you enter a command. After "su -", you can either run "/sbin/fdisk", or just "fdisk".