Basically, there's one or two extra steps you need to do for the H300.
You have to download a firmware update from iriver and put it somewhere on your player you could find it. Then run RButil. There will then be the extra steps during the install. RBUtil SHOULD tell you what these steps are, but basically when you have it do the full install, you have to do a couple things to finish. After running RButil you have to boot up your player, into the original firmware, and tell the original firmware to do a firmware upgrade (I'm not sure where this option is). If you have a US player you may need to reboot and do this again.
If at any point you aren't sure *exactly* what to do, don't click on anything, but ask a specific question and someone may be able to help.