The RadEditor has its own file manager uploader thingy, I guess that is what you mean by document manager. All we do is set the root directory in our RadEditorAdapter depending on user roles the root directory may be
/Data/Sites/[SiteID]/ for admins or content admins
/Data/Sites/[SiteID]/media/ for others (make sure this folder exists if not that might be the problem)
/Data/Sites/[SiteID]/userfiles/[UserID]/ for less privledged users.
if the folders are empty it won't show any files.
I have not played around with RadEditor in quite a while so I don't know if it has changed much since previous versions.
Hope that helps,