Yeah, sorry about that, I never got around to making documentation for XGUI. Using
maps.lua (lines 6, 176) as an example:
local maps = xlib.makepanel{ parent=xgui.null }
-- ...
xgui.addModule( "Maps", maps, "icon16/map.png" )
These two lines should be the bare minimum you need to get a panel showing up. Note that the addModule function has a fourth parameter that describes what access level you need to be able to see the tab at all. Everyone should be able to see it if it's not specified. Here's an example from the groups tab:
xgui.addModule( "Groups", groups, "icon16/group_gear.png", "xgui_managegroups" )
If you've set this to your own custom access level, you'll have to register it with ULib in serverside code, like so
sv_groups.lua (line 6):ULib.ucl.registerAccess( "xgui_managegroups", "superadmin", "Allows managing of groups, users, and access strings via the groups tab in XGUI.", "XGUI" )
From there, make sure your group has access to this new setting. Let me know if you still can't get it to show up after all that!