The Offsets feature took a second for me to figure out too. If you want to edit a line manually, click the box (say, the Y 1 box), and press Backspace to clear the number. Then type whatever number you want.
In order to use a line, you have to double-click the cell in the USE column. It's the second one from the right. Then click the Use button.
To save the current position, click Store Current Position and type in the row you want to save it in.