go to that file double click the xray icon and it should open maya
open script editor (bottom left of screen)
it should say
toggleXray;
selct this and middle mouse button drag it onto what shelf u want e.g. cutom.
on that shelf it shold have an icon sayin mel
and that icon will toggle - from there u can edit it so it has a diff pic or wording
hope this helps
reply if u get stuck