Sounds to me like you're missing the Database portion of the equation at least but I'm not that familiar with the Web Platform Installer, I typically install DNN manually, it may install some things for you not sure.
DNN requires a database, commonly used is Microsoft SQL Server 2005/2008 the Express edition is free.
If you check your programs and do not see Microsoft SQL Server you could follow the steps below, if it is installed make sure you can launch the client tools and connect to the database to make sure it is running if it is running check the Security mode (explained below), otherwise try this:
Download and install the Database and Client Tools from Microsoft's website.
When you install the Database Server it will ask you among other things if you want to install the default or a named instance, either one works, you just need to know how you chose to do it, either way note the instance name.
Once the DB is installed run a Windows Update to check for any security patches, run all that apply, a restart may be required.
Working from memory these are the major steps remaining:
- Launch the Client Software "SQL Server Management Studio" connect to the instance you created upon installing the server.
- Right click the Server Instance, go to Properties > Security make sure "SQL Server and Windows Authentication mode" is selected, click OK.
- Create a new blank Database, Right Click Databases > Enter a name leaving all other defaults.
- Go to Security > Logins, create a new user, Select "SQL Server authentication" assign a password, Uncheck all but "Enforce password policy", Default database is the new blank database you just created > under User Mapping on th left select the new blank database and give the new user "dbo" permissions.
- Go back to your DNN portal and enter the Server\Instance Name, Username & Password for the database user and if everything is running it should connect and you can now proceed to install.
Good Luck!
Josh