Press ALT + P in your mirc, that open the mirc editor, and P stands for Popup, now go to the tab "View" and pic the popup you want, i would guess its the Nicklist you want to edit.. type in your own slap there. it looks somthing like this
Slaps
.Slap1:/me slaps $1 around a bit with a pinguin
.Slap2:/me slaps $1 around a bit with a computer
and so on..