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"