If "Paste" wasn't a highlighted option, then apparently you had not copied anything to the clipboard. Of course nearly everything can be done with the keyboard, also, but I'll only talk about using the mouse. The first step is to move the arrow with your mouse to the beginning of whatever you want to copy and quote. Then hold down the left mouse button as you move the cursor to the end of what you want to copy. That should highlight that material. Release the mouse button, move the arrow up to "Edit" (top left of screen), click on "Edit", then click on "Copy". That saves what you want to quote until you're ready to "paste" it in your reply.
And being a computer dummy myself, a number of our TBN members have helped me on several occasions.