Here it is in pictures...
Hit the Quote button on the right side of the comment you want to quote...

That gives you this:

Then type your own text where the arrow shows below the quoted text.
If you want to only quote a small amount of text, I prefer to copy the text to quote...
Highlight and and copy it (ctrl+c on windows)

Then in my new comment, I place the cursor where I want the quote and press the "INSERT QUOTE" button on the toolbar

and finally paste in the text (ctrl+v on windows)

and then finish typing my message around it.