The permissions need to be handled on the web server, this is not something you can do from with DotNetNuke (although some hosting company controls panels allow this although still outside of dnn, probably not the case for most though).

The Users are different from the Hosting management software.  The IUser on mosttime is the Annonymous user of the IIS but there is also a user for the application pool and this is very different by each ISP.  The User must have create, change, write, execute, read permissions for each physical folder, subfolder and files in your DNN root.