I am trying to make an ASP.NET 5 site use visual studio 2015 preview, and i want to add dll at local file system to the ASP.NET 5 project. But i can\'t find this option, Is
You cannot add direct reference anymore, you would have to create your own nuget package containing it.
See: http://forums.asp.net/t/2002302.aspx?Adding+a+non+nuget+reference+to+a+vNext+project
As for the why, it is really easier to manage dependencies with nuget, download your sources anywhere, and with a single command (kpm restore) all nuget packages necessary will be downloaded.