Right-click on the "Edit DOS levelpak" window, and then from the context menu select "Add Editor Level". The submenu then has options for you to replace the selected level in the level list with what you have in the editor, or to add the editor level before or after the selected. Don't forget to save the levelpak afterwards! (Again, right-click to bring up context menu and then select "Save levelpak" or "Save levelpak As...", the latter to save to a different DAT file from the one you opened.)
You can also select "Add LVL file" instead of "Add Editor Level", which does the same thing except it uses the level from the LVL file you selected (it brings up the standard Windows file browser for you to pick the file), rather than the level currently opened in the editor.