Roundcube

I've been trying to install roundcube using the method described here, but I haven't been able to log-in successfully. To help diagnose where I went wrong let me show you the changes to the main.inc.php, and the db.inc.php.
For the db.inc.php (no .dist i removed that)
mysql://user:pass@mysqldatabasesubdomain.krusnix.com/databasename

For main.inc.php
$rcmail_config['default_host'] = 'mail.krusnix.com';

// TCP port used for IMAP connections
$rcmail_config['default_port'] = 38;

I have roundcube set up for my domain on DH and it works great.

Your database connction stuff looks correct.

Try port 143 for IMAP connections, instead of 38. So:
$rcmail_config['default_port'] = 143;

For SMTP, try port 25. So,
$rcmail_config['smtp_port'] = 25;

Now, as long as
$rcmail_config['default_host'] = 'mail.krusnix.com';

Your users should be able to log into Roundcube simply by entering their mail user name and password. So,

USER = krusnix (instead of krusnix@krusnix.com)
PASS = [krusnix password]

Another possibility is that your DH user-name and mail-name may be different. For instance, I have a DH username 'support' that recieves email at the address 'help@mydomain.com'. So, when I log into roundcube, I have to log in as 'support' not 'help', even though the address is 'help@mydomain.com'.

You can check this by going to DH-Panel>Mail>Manage Email. Then, find the address in question. The mail-user login name is in bold on the second line.

For instance, the entry for my user is:
help@mydomain.com
"Support Contact" support
17 MB / 200 MB (9%)

So, the mail-user login name is "support".

Hope this helps!