As far as im aware of, the copy and paste command for mIRC is the same as it has been in the past, there is no further setup.
Just left click and drag your cursor along mIRC's screen, when you let off of the mouse button, the highlighted information is attatched to your clipboard.
This being different than say a regular HTML screen or textfile where you have to right click and select copy, in mIRC you dont have to click 'copy'... the information is just appended to the clipboard as you highlight it on your window.
Then just right click into your editbox, scroll down, and click paste.