I know this question exactly looks like “No required driver detected for unity” on Dell Mini 1010, but like the title states, I'm not using a Dell Mini 10, but an Apple iBook G4. From what I understand of the associated question, it seems like a graphics device driver issue. So, where can I find such a driver ?
Unity requires accelerated graphics. You have ATI graphics and a PowerPC CPU. This configuration is not supported by the official drivers. However, the open source drivers should work. These drivers are built in but you may need to enable them.
Instuctions on how to do this and other information can be found of the Ubuntu Wiki.
This should allow Unity to run.