Yeah, the Option key should work.

[*]Target mode your iBook, with the DVD inside, and attatch it via Firewire.
[*]Restart the B&W
[*]Hold down Option during startup
[*]Enter the Firmware password if there is one (it'll bring up a little text box if there is)
[*]Click on the disc you want it you start from (in your case the DVD)
[*]Installer should run normally

Should be pretty simple if everything is as it should be.