The easiest location to put them is in the main mIRC directory.
If you're running Windows ME or XP, it's usually C:\Program Files\mIRC
If you're running Windows 98 (or earlier), then it's probably C:\mIRC
In any case, you can find out by starting mIRC, then typing into any mIRC window
That will show you the exact location of your mIRC directory.
Just remember (or write it down) then copy the files into that directory. If you have any further problems, don't hesitate to let us know