I would like to add it to the default menu (mouse right-click)
Like I want to right click on a folder and open it in the terminal. I don't want to have to find the directory in the terminal.
You first need to install the package nautilus-open-terminal from Software Center or Synaptic Package Manager

Then goto command line and type
nautilus -q
Then open nautilus and you can find "Open in Terminal"

If you want to assign a keyboard shortcut to this action, please consult the following Q&A: