You shouldn't just "download" the TAR file - you should INSTALL it. Use the following CLI command to install the WEB interface:
archive tar /xtract tftp:///
If you copied the TAR file into your switch, then, most likely, you do not have enough memory to install (unpack) the content. Prior to installation you should delete TAR file.
After you have your TAR file extracted, you should be able to get the web access to the graphical interface.
If you want to give different levels of access to your admins, you should either create local user accounts with a different "level access", or configure RADIUS/TACACS+.
Good luck,
Mike
CCNP, CCDP, CCSP, Cisco Voice, Security+, etc.
http://www.ciscoheadsetadapter.com