Qwt is a additional library which allows you to add graph in your qt program.
I assume that you have already installed Qt, and you have basic skills in programming in C/C++.
First you need to get qwt from source-forge : http://sourceforge.net/projects/qwt/
Then you extract the files.
Here comes the tricky part. The files you downloaded are not compiled.
So you must compiled them. To do so you must start the Qt prompt console.
You can find it on Windows in :
Qt 4.7.3 for Desktop (MinGW)
Then send the command that are on picture 2.
Congratulation, you perfectly compiled the library!