If you
install eclipse newer version i don' think so you need to configure
mercurialeclipse. I tried to install the new Eclipse and it successfully
build NS-3 on it. Are you sure you configure your NS-3 correctly ?
Please find the attached screenshot, which might be helpful to
configure.
Please
create a new project with C++ and browse to your NS-3 folder. Then
click on the project and right click on it and go to properties and make
the necessary changes shown in screenshots. I hope this helps.
After
doing that Please configure External Tool configuration (you can do
that by going to Run->External Tools->External Tool
Configuration).
Apply necessary changes and all done.