Ah, if it appears and then disappears that probably means that there is something that the program expects to find within the OS but that it can't find. How old is this Mac exactly? If it's more than 2 years old, it might still have Panther installed, with which Terragen is not compatible. Click on the little Apple menu at the top left, click "About this Mac," and if the operating system is below 10.4, you can't run Terragen on that Mac.