NONE!  This is new with MPNews 5 which was released in July 07.

You and your users do not need to download, install, or use anything but a Web browser to administer or access your NewsgroupHosting.com account.

As administrator, you can login at http://admin.newsgrouphosting.com/~<accountname>/, where <accountname> is the name you provided in setup.

Users will login with http://<accountname>.newsgrouphosting.com/

Users can use a newsreader like Outlook Express (not included with Outlook) or other various commerical newsreaders.  That link is news://<accountname>.newsgrouphosting.com/