Read through the whole wiki - the "configuration" section is after the "frontends" section where you quoted code from. It's pretty step by step and worked for me although I kept missing things in it and took me several tries to get it (its written pretty minimalistic as is most arch wikis - not complaining mind you).
Personally, I found network manager + arch + openbox to be a constant headache so I went with wicd
sometime back - it seems stabler and works well with openbox - you might want to consider it. I haven't been using network manager for over a year now so I don't really know (given the speed of change on arch) what its like anymore.