Hardly understand derivations at all

After looking up direct derivation, it seems you want this:

1) P→(Q→R)
2) (Q→R)→S
3) ∼S
4) show ∼P
---------------
5) ~(Q→R).........2,3,→O
6) ~P..................1,5,→O

Im not exactly sure what →O means. I've entered it into the problem but it says that there is no rule named that. If you could tell me what you mean by →O I could probably figure out what to put in the problem.

Look at page 162 of this:
http://courses.umass.edu/phil110-gmh/text/c05_3-99.pdf