You mean you don't have the /window command in your mIRC help file? I suggest redownloading mIRC from www.mirc.com then.

In brief:
/window @window
Makes a window called @window

/window -d @window
Makes a window called @window on the desktop

/window -c @window
Closes the window called @window

There are many many switches, redownload and install mIRC then type /help /window.