Please follow these instructions if you are on a Mac and you get a popup stating "Steam needs to be online to update", even though you are already connected to the internet.
Mac OS X (all versions)
• Open Finder.
• Click the "Go" menu in the menubar.
• Press and hold down the "Option" (Alt) key.
• Click the "Library" item in the menu.
• Go into the folder named "Application Support" then "Steam".
• Delete the file named "registry.vdf"