Hi,
There are a few common issues when using the platform installer. Your SQL Server must be configured for Mixed Mode Authentication so you can use sql users instead of windows users.
In order for it to create the db for you, you must know the sa user password (or another admin user). It does not validate this so if you enter something incorrect it just fails in an ugly way.
When you enter the information to create the new db user, is where things often go wrong and the error messages are not helpful. What you need to know is that it enforces some password rules, the password must be at least a certain length, it must have at least one upper case, one lower case, and one numeric , and one special character, a password example that meets the criteria is $Secret123
Note that the platform installer does not install the source code, it only installs a pre-compiled package, there is no C# source code included and no project or solution files, so when you open it in VS, it opens it as a "Web Site Project", but really our actual source code is not a Web Site Project but a lot of different projects in a solution and our main web project is a "Web Application Project" which has a .csproj file and lots of C# code files. To work with our actual source code you do need VS 2010 and you should get the code using TortoiseHG.
Note also that our package in the Web App Gallery is currently a package for .NET 3.5, once the Web Platform Installer 3.0 comes out of beta we will be changing our Web App Gallery Package to .NET 4. Our source code is pre-configured for .NET 4.
Hope it helps,
Joe