Normally when you get that error message what actually happened was that the download didn't fully download, and so the archive is invalid. Try downloading those again, and if possible watch to see if it completes. I'd also recommend letting one finish before downloading another (download one at a time) and see if that helps. Also check the size of the files that aren't opening with the size listed in the download section. If they don't match it's a pretty good indication that they didn't completely download.
EDIT: I just tested with the Raptor Pack Easter Pack and it downloaded for me and unzipped without a problem. It did take a bit to download, so it's possible it didn't finish for you - especially since it seems you are having troubles with the packs, which are bigger and can take longer to download.
EDIT2: Moved this to the proper forum section.