How do I reference a namespace in another .cs file?

Question

So I'm for one of my own .cs files, I'm trying to use the TestPDF Namespace from the last post here:

http://forums.asp.net/t/1886131.aspx?Convert+HTML+to+PDF+using+iTextSharp

which is contained within HTMLToPDFBuilder.cs. So for my someFile.cs I try to use the TestPDF namespace with using TestPDF; However, it says

The type/namespace could not be found [...]

How do I add a directive/assembly reference to this namespace within the file? I'm using VS2013 and tried right clicking my project stub in the solution and did Add>Reference, but it only takes .dll-esque file extensions, not .cs ones.

Answer 1

Right click and then 'Add Existing Item'. Answer courtesy of @Savanna King.

Answer 2

Add your DLL to your references library, if you still get this error then CLEAN your solution then REBUILD.