Assuming you want to use the same custom _clientui.ico file that you used on your Windows installs, you need to convert the .ico file to a .icns format. Then you can replace the default icon at /Library/BESAgent/BESAgent.app/Contents/MacOS/BESClientUI.app/Contents/Resources/BigFixIcons.icns with your custom icns file. Restart the agent and it should now appear in the menu bar with your custom icon.