Rich I've just installed Roundcube to see if there were any problems. It seems to have installed okay (I had to do some manual fiddling with the database and use a test db because DH takes too long to make the subdomains available . But that was just local to me)
Still if you've followed the wiki exactly you should be almost there.
In the config folder apart from .htaccess are the two include files that you have to update as per the wiki.
What I had to do though was copy
db.inc.php.dist to db.inc.php
main.inc.php.dist to main.inc.php
since my tarball only came with the two distrib files. (Perhaps the wiki needs updating there?)
The rest is just following the wiki in order. Ignore some references to line numbers they appear to be a bit out.
If you still have a blank page carefully check you have entered the MySql username correctly in db.inc.php.
When logging into Roundcube I didn't use the full username address just the m number and password.
First impressions of RoundCube? I'll probably stick with Squiddle