Have you installed an OS yet? You need to finish at least one installation before you can boot from any disk image.
To install: Make sure you have a virtual slave ide0 device configured as "cdrom" apart from your master image. So dive into the configuration file with wordpad (the file officially uses a non-windows line end which wordpad supports) and add something like (if it doesn't exist):
pci_ide0_slave_installed = 1
pci_ide0_slave_image = "d:\pearpc\cd1.iso"
pci_ide0_slave_type = "cdrom"
This would boot the cd image "cd1.iso". For MacOSX you only need CD1 if you aren't planning on using printers or additional languages. Also, the installation would top out at about 890 MB, leaving just enough room on a 3gib disk image for some additional programs.
Just remember that this is only to get the feeling of mac os. If you want to find out another way, and have some left over cash you want to spend: Buy a second hand G4 AGP (minimum!) (power/i/e-)mac around 533MHz and with 512 MB RAM minimum (for <533, >700 benefits from a little speed so can run well with less memory) with MacOSX (be sure to get the official cds/dvd). Though this is a little off topic... ;)