From your toolbox, choose the constrained line tool (|--). Use it to make a continuous line where you desire the dotted line. Then, using the arrow tool, click on the line to select it. Now click on ELEMENT at the top of the screen, then STROKE, from its drop-down menu. Here you'll find a box with several linetypes to choose from. As soon as you choose one of these, the continuous line will change to dotted, dashed, or whatever you chose! If you want a thinner or thicker dotted line than what you have to choose from, merely click on CUSTOM from the top of the linetype box, then you can choose a linetype PLUS its thickness.
To Erase, merely select any object with your arrow tool, and hit your BACKSPACE key.