If every one reads manuals, then we will have very less questions here and less reputation too 😉
To make nomenclature you have to pdflatex the .tex file and then run this command:
makeindex -s nomencl.ist -t "<file name>.nlg" -o "<file name>.nls" "<file name>.nlo"
where <file name> is the name without .tex extension (myfile if I have myfile.tex).
Then again pdflatex myfile.tex.
Now the question is how to run makeindex with all those arguments in texstudio.
You can create a user command for this. Go to Options → Configure TeXstudio. In the window that appears, choose Build tab on the left side. Search for User Commands. (You may have to click Add button with a green +). Type
just after user0: (0,or 1 0r 2, depending on how many use commands you already have) in the first box. In the second box type:
makeindex -s nomencl.ist -t %.nlg -o %.nls %.nlo
Now OK it. Illustration below.
Now you have a menu under Tools → User with a short key also.
Now run pdflatex → Make Nomeclature → pdflatex. Hope things are now clear.