mm, actually you still cannot use fdisk as an unprivileged user. the best thing is to use su -
instead of su
Anyways, to mount the partition, use the following command as root
mount <partition> <mount-point>
also look into /etc/fstab if you want to have it automount at boot time.
Feel free to mount it wherever you wish, just don't mount it over a directory with data in it.