Keep getting permissions error on IIS7

I am hoping that you can help me.

We are trying to set up FCKeditor on this machine and we keep getting the error 103 - You have no permissions to create the folder - from the diagnostic tool.

You can see the errors that occur at this URL:
http://66.29.196.12/fckeditor/editor/fi ... /test.html

We tried giving the IIS_IUSRS write permissions and nothing seems to work.

It seems that we are adding all the permissions needed but it is not having any impact. Could it be that we are should be giving something else permissions here?

Any thoughts that you might have would be welcome.

Many thanks

Richard