How do I match/play a particular person or a friend? (ICCforMac/Windows)

Toolbar above: Play Icon  > 'Create Challenge' window opens > Fill in the different fields for time control, type of game, etc. Choose the tab 'Challenge Friend' and fill in his username > Press 'Send Challenge'. A match request will be issued to that person.
 
To clear your match offer go to the menu above: View > Console or click the Console icon in the Status area at the bottom left.  Type on the command bar under your console: match

For more information, including the command line method, CLICK HERE.