It is wherever you installed mIRC. By default, it's c:\program files\mIRC\ (or c:\program files (x86)\mIRC\ on a 64 bit computer). You access it like any other program. Open on the start menu and click on Computer and browse to the folder.
If you have mIRC running, you can find where it's installed by typing in mIRC: //echo -a $mircexe
If you want to know where mIRC's settings are stored, you can type in mIRC: //echo -a $mircdir
