It may be browser dependent or some other issue that causes the Paste option not to appear when you right click in the box where you type your reply. I just tried both the quick reply or post reply and it worked for me using IE.
The control + C (copy) or + V (paste) are very good things to know since sometimes they are the only way to get that done, and are pretty univeral within Windows. I work some in a Unix environment where control-C means something else, and there I have to use shift-insert or control-shift-V to paste.