From their post on the official forums:

Starting on Monday, February 1st, Mortal Online opens it's doors to the public with an open beta.
We invite the whole gaming community to step in the wonderful world of Nave. Join Mortal and experience a fantasticly immersive, beautiful and brutal sandbox MMORPG.

To join the open beta, simply create a game account (a forum account will automatically created).
Download the client, install and play.

The open beta will run on one cluster server based in Stockholm (Sweden). Please keep in mind that, depending on the onslaught of new players, we will have to cap the server at a certain point.

There will NOT be wipe before open beta.

Related links: 

• Open Beta Thread -  http://www.mortalonline.com/forums/31725-open-beta.html
• Create an Account - https://account.mortalonline.com/account/new.php
• Mortal Online Official Site - http://www.mortalonline.com