Error in web.config

I am using Visual Studio 2013 Express.  I created a web site and it worked great.  I then added some code I downloaded that allows me to create Forums on my site.  They said to create a folder named Forum and dump all the code I downloaded minus the bin directory into the new Forum folder.  I did that.  But when I compile the web site, I get this error:
It is an error to use a section registered as AllowedDefinitions='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS.
In folder Forum, there is a web.config file.  When I click the error message in VS 2013, it displays the forms authentication in this web.config file. Here is the web.config code:
<authentication mode="Forms">        <forms name=".YAFNET_Authentication" protection= ...