A couple of additional comments...
1) If you're not using some kind of network hub (ie: just putting an ethernet cable directly between the PS3 and Mac), I'm not sure if either the PS3 or Mac will do the required 'cross-over' of the transmit and receive pairs of wires. Most old network techs like me used to carry around what was called a cross-over ethernet cable for just such an occasion (usually for testing in our case). But, I'd suggest just using an ethernet hub and 2 normal ethernet cables. Maybe someone here knows if you need just a normal cable could be used (and either the PS3 or Mac does this automatic cross-over... I've not tried it).
2) As other's mentioned, you'll have to get them both on the same subnet. In other words... 192.168.1.x, or 10.10.10.x where both your Mac and PS3 have a unique number for x, but the other numbers are the same.
The easiest way to do this would be to use an actual router, such as a Linksys that people would use for an Internet router. It wouldn't have to be connected to the Internet necessarily (but could be), but it will take care of assigning DHCP addresses and creating this local 'home' network. But, you should be able to do it with your single cable if that is really what you are determined to do.