The Gmail app will not allow you to add labels to messages. The "Enhanced Gmail Plug-In" for the inbox on your device will. Make sure you are using that icon on your BlackBerry to check your Gmail inbox instead of the Messages icon. Highlight a message/conversation, hit the BlackBerry menu button, select Labels, then tick the box for the label you want applied to the message. Press the menu button again, select Save, and you're done. Hope that helps!