When you reply, you do so in message text box. Directly above that text box there are 2 rows of icons. Look at the lower row (ie immediately above your message box. The icon at the far right end looks like a comic bubble.
1. First find your quote from another message and highlight it and then copy.
2. Open up your reply message box
3. Click on the speach bubble and quote brackets will appear on both sides of the text cursor. Right click and hit paste. hey presto
Hope this helps