If you have downloaded a version older than 1.3 you need to do the following for making and installing 
the package. Type in your console:

'tar -xvzf netstat-nat-1.2.1.tar.gz'
'cd netstat-nat-1.2.1'
'gcc netstat-nat -o netstat-nat'
'strip netstat-nat'
'cp netstat-nat.1 /usr/man/man1/'

And you are ready to go.