Hi Rosil,
I have tried to use webmatrix once and did like the setup it did. So I do not recommend using Webmatrix to setup DNN.
On that sense I won't be able to help you much other than pointing you to the videos we have here on how to install DNN which takes the manual route but empowers you to understand what is going on each step of the way. This is what I suggest, change your installation strategy:
How to Install DNN
Let me know if you have any problems with that installation process.
Best regards,
Aderson