Adapters that make USB keyboards work with PS/2 ports are simple and common; they are just wired to change the shape of the connector. The USB keyboard just needs to be designed so it can work with those older ports (through that adapter). However, for PS/2 keyboards that were made before USB ports, they are not designed to work with USB. So I don't think it will work unless you have a more complex adapter... one that may need a driver. If it needs a driver, you will probably not get Mac compatibility, because Macs never used PS/2 ports. The older Mac serial ports were called ADB ports, and to use an older Mac (non-USB) keyboard on a Mac USB port, you have to get a special (non-simple) adapter called the Griffin iMate. So you would need something equivalent to the the iMate, except for PS/2 keyboards, that works on a Mac (with maybe a need for a Mac driver).
If would be far easier to get a USB keyboard and USB mouse. Generic ones cost less that $10 for each, which is probably less than the cost of a Griffin iMate -type device.
However, using Google, I did find devices like this one that claims Mac compatibility.
http://www.usbgear.com/USBTO_PS2CONVERTER/
This is obviously not a simple $5 adapter, but this one adapter does connect both the PS/2 mouse and keyboard to one USB port, which will save you a port (just like with a REAL Mac USB keyboard and mouse).
