Be sure you have created an account at www.fantasyrealmonline.com/register.aspx before proceeding.
After all Moon Haven content has been downloaded and patched the content will be loaded and you will be presented with an intro screen. To login click the stone login text at the top of the screen, you will be presented with a login dialog. Enter your username and password in the dialog box, then click the "Ok" button.
Note: all credentials are sent to the server using industry standard encryption