Hi Kenn,
I retested on 6.4.0P2 for you just now (allready tested before on 6.4.0P3).
When I do tests like this, I always try to keep the FCKeditor folder within my project in the same version as the SDK I'm working with, so I obtain the folder from that version. I just assumed that the FCKeditor folder would be within the Catalog source download, but as you say it is not, you will have to obtain it from SVN. For getting the SVN sources, check here. You would not be obtaining the trunk version (the current being-worked-on version in SVN, as that would be P3, but I also tested this, that P3 version of FCKeditor folder works fine in P2 too).
In the SVN checkout, locate the FCKeditor folder and copy the contents to your virtual folder, so that you have the structure Yourapp\FCKeditor and Yourapp\FCKeditor\editor.
Now on your form, drop a FCKEditor from the toolbox to the form. Note the BasePath property. It should include 6 sets of "..\". In my case it only contained 5 so it didn't work, and I had to add the 6th.
Register your FCKEditor control in web.config, in my case it was:
<Control Type="Gizmox.WebGUI.Forms.Editors.FCKEditor,
Gizmox.WebGUI.Forms.Extended,
Version=3.0.5701.0,
Culture=neutral,
PublicKeyToken=85eae29607c9f5f3" />
And that should be it. I tested on IE8, FF3.5 and Chrome, and I didn't see any immediate problem. Didn't test much though, just made sure it did show up runtime.
Please let me know if any steps are unsure ...
Hope it helps,
Palli