Can I receive entries sent by mail?

Yes!

Login as an admin, click into a show and click [add entry]. Fill in the first and last name of the entrant (and email if you have it) - or just fill in part of the first and last name. Click the 'Find & Add »' button and you'll get a list of already existing entrants that match what you entered. Select an entrant from the list or click a link to create a new entrant. When creating a new entrant, you only have to enter a first and last name. If you want the entrant to be able to login and view their entry, enter their email address, create a password for them and send them an email with that information.

After you've selected or created an entrant, the process is the same as the entry process for entrants.