Choose Lines from the Edit | Current Type list.
Execute Edit | Add Objects to enable the Add Lines operation.
All of the Points are displayed as bold Points so that you can see them more clearly.
Click near the first end Point of the Line to select it.
If you select the wrong first end Point, click on it again to un-select it, and repeat the step above.
Click near the second end Point to add a new Line to the model, and a new row to the Lines spreadsheet.
WARNING: When the Add Lines operation is enabled, a new Line will be added to the model each time you click near two different Points in succession.
Execute Edit | Add Objects again to terminate the Add Lines operation.