The full installer is unfortunately no longer compatible with the current running version of heRO. You'll have to download
Ragray, since heRO uses some Renewal, or at the very least their graphics. Once you've downloaded it, install it and use the start menu list for Ragnarok Online to full patch all 3 versions (there should be Korean, Korean official, and Korean Renewal).
You'll also want to download the
mini-installer, after you've installed Ragray. Install the mini-installer to the same folder you installed Ragray in, and if everything goes along smoothly, you should be able to get into the game (after quite a bit of patching).
I'd say at least set aside half an hour for everything to patch. Hope this helped you, if you have any more questions you can ask here.