Click on the 'My Labs' link at the top of the page.
Click on the lab and select 'Schedule' from the pop up menu:
A new browser window opens with a scheduling interface present:
Select the day and time that you would like to reserve the lab resources
Click on 'Reserve this Interval'.
Your lab is now scheduled.
The schedule window disappears and you are taken back to the 'My Labs' page. You will notice now that the lab has a 'Scheduled For' time and date under it.