Why can't Visual Studio find my DLL?

Specifying the path to the DLL file in your project's settings does not ensure that your application will find the DLL at run-time. You only told Visual Studio how to find the files it needs. That has nothing to do with how the program finds what it needs, once built.

Placing the DLL file into the same folder as the executable is by far the simplest solution. That's the default search path for dependencies, so you won't need to do anything special if you go that route.
To avoid having to do this manually each time, you can create a Post-Build Event for your project that will automatically copy the DLL into the appropriate directory after a build completes.

Alternatively, you could deploy the DLL to the Windows side-by-side cache, and add a manifest to your application that specifies the location.

I've experienced same problem with same lib, found a solution here on SO:

Search MSDN for "How to: Set Environment Variables for Projects". (It's Project>Properties>Configuration Properties>Debugging "Environment" and "Merge Environment" properties for those who are in a rush.)

The syntax is NAME=VALUE and macros can be used (for example, $(OutDir)).

For example, to prepend C:\Windows\Temp to the PATH:

PATH=C:\WINDOWS\Temp;%PATH%

Similarly, to append $(TargetDir)\DLLS to the PATH:

PATH=%PATH%;$(TargetDir)\DLLS

(answered by Multicollinearity here: How do I set a path in visual studio?

try "configuration properties -> debugging -> environment" and set the PATH variable in run-time