How to run Apache benchmark load-test in Windows?

I do agree, PATH ENVIRONMENT setup manually

C:/YOUR_INSTALLATION_APACHE2/bin

Path to environment on control panel system settings.

I don't know the path WampServer is installed to, so I'll just show you how I do it under WampDeveloper (which is what I use).

Run cmd.exe.

Inside...

C:
cd \WampDeveloper\Components\Apache\bin
ab -n 1 http://www.example.com/

To answer your question, you type it in the command line changed to the bin folder of your Apache folder since this is where ab.exe exists. If this folder location is in the system path, you can also just type it in anywhere (without changing paths in cmd.exe).

This 1st line changes the drive letter. Then second the path (aka working directory). The third runs ab.exe.