Once the meeting invite opens up, drag your file you want to attach into the blank gray area to the right of Duration. Your attachment will then appear in a newly-appearing attachment line below the start and end time for the meeting.
Interestingly enough, you can’t add files to Outlook 2011 appointments. If you need to add a file attachment to an appointment, click the Invite button in your Appointment window. The appointment will then turn into a meeting invite and allow you to attach files. Invite your own email address and hit the Send button to add the event to your calendar.