" How do you know this contains information, and that isn't just a random design generated by natural forces of erosion?"
Because we have numerous examples of people writing on stone throughout history. Because erosion wouldn't leave such detailed marks. We have no examples of non-man made writing.
"If you are an evolutionist you will not even attempt to entertain an alternative solution implying intelligence."
The analogy falls flat. We have a means to discover who wrote the Rosetta stone; in fact, we have (not who specifically but what people). Archaeologists didn't say *The Designer wrote it*; they used physical evidence to determine how it was made. There is no way to investigate ID proposals for the origins of the universe, for the origins of life, for the evolution of life. The two examples are completely different. The hypothetical Designer didn't leave a calling card in our genes.
Because we have numerous examples of people writing on stone throughout history.That's a bit circular logic. We've found plenty of artifacts that have contained writing unknown to us, but nonetheless concluded that it was in fact writing, based on the immediately discernible fact that no forces of nature would have produced them.
Because erosion wouldn't leave such detailed marks.
In other words, because we ruled out natural causes. That's what ID sets out to do, by saying that there are organisms and features of organisms that could not have arisen through any natural cause that can be identified. Maybe they're right, maybe they're wrong, but it's not "unscientific" to pursue that line of inquiry.