First, unzip the download. Then move the whole folder to Public Documents/Stardock/IconPackager. (or unzip to that location). Then from within the IP Main UI, do the add package from disk thing. IIRC, that has worked for me. If you just add it from wherever you downloaded to, or extracted to otherwise, you will have those issues you mentioned with some packages.
If that doesn't work, you might try PM'ing Ace Matrix. He is familiar with the problem. (and a mighty fine icon artist too)
EDIT: I just tried it with Metal Storm, and this method is what works for me on XP and Win7. Not too sure about when running Bootcamp.