La soluzione è riportata su questa discussione sulle comunità di supporto Apple.
Segnalano diversi modi per farlo.
Il modo più semplice sembra essere:
Insert your Tiger/Leopard/Snow Leopard DVD into your drive. Once the
DVD loads, open the "Optional Installs" folder and then the .mpkg of
the same name. This will launch an Installer. Click through until you
get the option of choosing what you'd like to install. (this is after
the Licence Agreement part)
Open the "Applications" by using the tick box and triangle for the
drop down menu in the panel of software to install, simply choose
iChat (only) and let the installer do the work.