In Lion, the Library folder is hidden so you won't see your Steam folder. If you don't want to mess around with the registry or enter Terminal commands, you can simply go to the 'GO' menu, select Go To Folder, and enter: /Library You should now be able to see the Steam folder, and the registry.vdf file inside that. Simply rename the file, or delete the full stop so that Steam doesn't recognise the file (you can delete the file, but best to rename first as if this fails, you might want to be able to revert back). Once the file has been renamed, you should be able to start up Steam and get back online. Note: if the above fails, try following all of the steps again but rather than starting up Steam, re-install it instead. Worked for me - I'm now back online.
Happy gaming! Steam has always been crappy, both on Mac and pc. Truthfully if steam wasn't the only legal way to play valve games I would just stop using it. I've had TO MANY problems. Here are a few. Offline mode not working when not connected to a network The Mac client crashing when started in offline mode on a Internet connection(Your problem) Some games not starting in offline mode even if they are single player games(Most recently Half Life 2). Steam crashing when I click out of it to another window(windows) Steam not maximizing after I minimized it These are only of the few problems that PLAUGE the Steam client in truthfully just shows how lazy valve is.
No wonder they can't count to three. I'm thinking of starting a website called STEAMBAY.com. It'll be like pirate bay but specifically designed to put valve out f business. Or at lest make them update their client so it not the glitchiest thing since Windows Vista.