I also downloaded the v3 RC1 version today and had the same problem.
If you open that file in Visual Studio, you will notice that it is not complete.
So
that I could simply get past these errors and onto the install process, I simply deleted that
file from the Project in Visual Studio, rebuilt the Project, and then revisited the install.aspx
page in my browser. Another 3 or 4 errors occured pointing to different files with pt-PT in the name.
After deleting each of these files from within Visual Studio, I would re-build my project and refresh my web browser. After about 3-5 files, I was presented with the DashCommerce install page and completed the installation successfully.
Before running the install script, I had already:
1). Installed the necessary components (Subsonic, Ajax etc)
2). Created a database on SQL Server with full-text indexing
3). Created an application in IIS pointing at the /web folder of the project