Let's break this one down a bit.
First off, did you receive the email receipt for your booking? (the outlook calendar attachment comes with this email). In order to receive email receipts and calendar attachments, you must tick the box for 'send a receipt?' on the booking form. If you're not sure if you ticked this box, try another booking making sure that you ticked this box.
Sometimes emails like these are considered to be spam by your spam filter and automatically deleted or quarantined. Check your trash or spam folder to see if it is there - you may need to add the sender's email address to your safe sender list.
If you did receive the email, you must double-click the attachment to insert the booking into your Outlook calendar.