When you open a dlg file, there are "nodes" that represent the lines the NPCs say (Red, called "Entries") and your possible replies (Blue, called "Replies"). If you want to edit a node, you'll have to click it and edit the preferences of this node at the bottom of the program. On the right there is a section called StrRef. Change this to "-1". Now you can type your text for the Node. When you're finished click somewhere in the section of the nodes. Your changes will be applied. Once you're done with the file save it.