MEOWCHiX,
I was actually going to mention something similar to that after the post before yours, but forgot I guess. ^^;
When installing the game via Steam (which I personally do not recommend due to various reasons, but will not go into details about here), one should only install via Steam. What this means, any existing install should be un-installed, and then a fresh install implemented via Steam.
The PlayOnline Viewer downloaded via the website will also not work with the Steam install.
As for registering the codes, it is done via the Square Enix Account Management System, as I mentioned in a previous post. No account management is available via the PlayOnline Viewer at all, and has not been for some time now (has it been over a year or 2 now?)
You will have to register at least one Content ID via that management site for your service account, directions to which are likewise mentioned in my previous post here.
I hope this helps~
Edit:
Do note that in cases where one already has the game and the expansions installed via other means than Steam, it is possible to gain access to the add-on scenarios by obtaining the codes via Steam, and registering them to the Service Account without the need to install anything via Steam.