It's not quite that simple with the liberty's.
In order to fit a new cd/dvd player you will also need to purchase an aftermarket double din facia with the seperate climate control (with the standard setup the climate control is actually integrated with the head unit)
These arent cheap, they generally go for around $300-$400, even more from some places. With a bit of luck you'll pick one up second hand.

If your not already a member i suggest signing up at forum.liberty.asn.au

Fyi i can confirm the macintosh cd players do not play mp3's

Good luck