Normally you use "Insert row" from the Insert-menu, or the popup menu shown when you right click in the ladder diagram.

You can also use the keys "Ctrl" + "Alt" + Down-arrow to insert a new line or "Ctrl" + "Alt" + Up-arrow to delete a line.

These methods insert a new line above the currently selected line, so none of these methods works if you want to add a new line below the last line in the rung.

Then you need to use "Enter" instead. It inserts a new line below the currently selected line

Make sure the cursor is not located on any contact or instruction (it will only open the settings window for that contact/instruction) and press "Enter".