OK, I figured it out. It took a while because the instructions on the page I mentioned are incorrect (I had to make changes to the user mapping to the app pool account, not NETWORK SERVICE). I just don't understand how something as basic and essential can be left broken. It seems the page hasn't been edited for over a year.
The video's at the bottom seem really old too (DNN 5.6?!!!). Surely it wouldn't take a ton of time to update them (and that would also be a good opportunity to check the instructions).
What I'd like to see is a specific page for a combination of environments with absolutely precise, fully validated steps. For example, WS 2008 R2 and SQL Server 2008 Express is probably a common combination and there should be a specific page for that. And maybe WS 2012 and SQL Server 2012 Express is another one. These are sufficiently different that they justify dedicated installation pages, IMHO, with the steps listed only those that are relevant to that platform.
As it is (on the page I mentioned) I have to parse the various versions of SQL Server in the text of the page, throw out stuff that isn't relevant to me and so on.
It's simple too much hard work figuring out how to install DNN and my guess is that lots of people have tried and given up silently.
Mark