Well, there's two possibilities here.
1) The ebook is packed inside the rar file, which means you'll need to extract it first. If so, I'd go with sleepy bob's recommendation of either WinRar or 7-zip to unpack it.
2) They packed an epub using rar instead of zip like they're supposed to. If that's true, unpack it (using the tools recommended in #1), repack as zip, then rename the extension to epub.
For now I'd just assume #1, and if that turns out to be wrong, go with #2.
