After I upgrade to new iMac, I could not login to steam.
I searched the web and tried various things and finally figured out the solution.
Every Steam Account ties to authorized machine. If you purchased a new Mac, you have to authorize it. Moving Application and Folders are not enough.
So, if you already have Steam installed on your new Mac, delete “Steam” application and all the related files using Uninstall utility such as AppDelete.
Then re-install Steam and launch it. The app will says “This is unauthorized and we will send you an e-mail” and let it so.
Check your e-mail from Steam Support. Copy the special access code and paste it in Steam Application.
You should be able to re-authorize your new Mac.