Another way, is to add all the icon files to PB
that is:
PB > Project > add files...
(I dropped them under the main group)
After that, fire up IB. The icons you added show up in the "images" tab in IB.
From there you can just drag and drop them onto the target buttons.
This way works for me. if I just update the icon field in the inspector panel, well... nothing happens. Could be related to my version of the Dev Tools (the latest).