Metal stud work is becoming more popular in new houses but does pose issues when fixing to walls .In your case with this unit it is designed to sit in the corner so you will definately hit stud work .Plasterboard fixings will not do you any good in this instance .Select a screw to be used ,the screws supplied should be fine and pre drill with a steel drill bit that is slightly smaller than the screw allowing the screw to self tap into the metal stud work .
Hope this helps
I use
these screws for drilling directly into metal studwork without having the need to pre drill ,they can be purchased at just about any supplier ,use a cordless drill with a PZ 2 bit in set on fast and they will go through no problem