this step is not necessary ,ri
 

this step is not necessary ,right! i can skip it .

i can not understank " $WM_PROJECT_INST_DIR ",what it mean .
regards

Yes, you don't need to compile
 

Yes, you don't need to compile OpenFOAM from source.

If you type in a terminal

cd $WM_PROJECT_INST_DIR

and press enter, you will enter the $HOME/OpenFOAM directory. $WM_PROJECT_INST_DIR is an environment variable whose value is "/home/username/OpenFOAM".

I think you need to learn some more about Linux or UNIX operating systems. You can find many guides online (for example: http://www.linux.org/lessons/beginner/toc.html).

Google is a good starting point to find many information.

Alberto