To create an HTML line click on the HTML item toolbar icon and draw an item - this will be rectangular for the moment but don't panic, we'll use the Inspector to make it the right shape. With the HTML item selected open the Inspector palette (if it isn't already) and go to the Dimensions area. In the Width field enter the width you want the line to be and in the height field type 1px or whatever height you want the line to be. Now give the item a color.
That's it, you now have a line.
Note that the item can be either a table or CSS layout item, it makes no difference to the overall look of the line.
To create a graphic line either do the same as above with a graphic item, or click and hold on the Graphic toolbar icon and choose Path. Click once on the page to start the path and then click twice where you want the path to end. To give the line a color you will need to assign a border. You can do this in the Appearance settings pane of the Inspector palette (see image).