When you enter My Account, simply select the Manage link found in the Out Of Office section of your dashboard to enter the Out Of Office area. You can also access this area by selecting Out Of Office tab displayed at the top of your account page. In this area you can enter the dates that you are due to be away - don't worry if you've forgotten to set up Out of Office and are already away, you can just set your ‘Out Of Office To' date and you will be shown as Out of Office to all users until that specific date.
You can also create a personal reply that will be shown in discussions that you are subscribed to while you're away. If you don't wish to create a personal reply, a 'default' message will be displayed instead. When you're happy with your choices, just hit the 'Set’ button and your Out of Office status will be set to activate according to the date(s) you have selected.