I'm a Mac user. when I've connected my Iriver Story to the Mac I open two windows in Finder, one for Story and one for wherever I have the books I want to load - usually in Downloads.
Under Story you'll find a folder called Books and, under that, one called English Books with your 100 free books in it. When you're there you can drag files from the other Finder window to the Story/Books/English Books folder and drop them there. Make sure the books you load have a file name of the form "Author Name - Book Title" or just "Book Title" or you won't be able to add a bookmark. DON'T use, for example, "Book Title - Author Name". I've written to Iriver about this and hope there'll be a fix some time.
Another thing about the Mac. When I eject the Story, it reconnects again within about ten seconds. Don't know if anyone else has this problem. You have to be very quick to pull out the usb cable if you don't want to get a comment about the disk not having been ejected properly. This problem doesn't exist on Windows (XP) or Linux (Ubuntu 10-4).