How do I add time to my opponent's clock? (ICCforMac/Windows)

Use the moretime command.

The format is: moretime seconds
When playing a game type on the command bar under your game chat console: /moretime 30   --   to add 30 seconds to your opponent's clock. 

 

  

Or type on the command bar under your console (menu above: View > Console): moretime 30  --   to add 30 seconds to your opponent's clock. 
You may add up to 999 seconds of time. You can't add time to your own clock. 

More information in "help moretime".