Send Fax is found in the left navigation - under the Extensions option...

 To receive a fax on your One40 number, you must have your number set up to "Play my main greeting"...

When you send or receive faxes it uses your plans lines

The currently supported file formats include...

Inbound faxes are displayed in your Inbox...

When your One40 Numbers are configured to Play Main Greeting, they can also receive inbound faxes and display them in your Inbox...