Yes, it turned out that somehow or other the directory that was supposed to be called subSilver had been changed to subsilver instead, and linux systems being case-sensitive, that meant it couldn't find it.
I'm not sure HOW the name got changed - other people in the phpBB support forums suggested it was probably either my FTP program or by unzipping program, but when I downloaded a different version of phpBB recently, using the exact same software, it didn't change the capitalization, so I don't know... Anyway, if you do get that error, check the name of that directory, and also the subSilver.cfg file within it, and make sure they have that capital S.
With regard to the MySQL setup, I've found a really easy way to manage MySQL databases is to install phpMyAdmin (http://phpwizard.net/projects/phpMyAdmin/). It gives you a graphic interface where you can not only create and delete databases, but also add tables and data within them, see what databases you already have, etc. Particularly now that DH allows unlimited databases, it's very handy. I know the web panel gives you some of that functionality already, but phpMyAdmin lets you get right inside the databases and see everything that's in there, set up new tables, etc. so it gives you more control.
One other thing worth noting - for anyone that's converting from YaBB to phpBB due to DH's recent announcement about the evils of the former, there is a conversion script on the phpBB site that will convert a YaBB board to phpBB - BUT it only works with the RC1 release of phpBB2, not the final release. So if you want that script you have to download RC1 from the phpBB2 sourceforge site, run the conversion script, and then upgrade it to the final version.