You can't really make the old man see it, its only a matter of luck! ! What I do is I check everyday .... and it will eventually appear.
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You have to ask a man in pacificlog town and ask if he can see mirage island. If he does sail east till there is an island. On top of it and beside there are the lechi berries. But I bet you'll never see the mirage island!!! It appears only when the ID number correctly, the same ID as the mirage island.
A man in Pacific Log Town will tell you whether or not he can see Mirage Island each and every day, or every 6 hours. Then you can surf there from Pacific Log Town.
When you have a corrosponding Pokemon number. Each Pokemon trainer has a certain number. Trade with friends to get more numbers and one of the numbers will be able to unlock mirage island
There is no "easiest way" just do the normal way. Normal way: Talk to the old man in Pacifidlog Town. Once he see's it go to route 130. Look for mirage island. Note: Mirage island is not that important it only has the rare Leichi Berries and in the grass all you will find is wild Wynat.
wait for the old guy in the house in southeast Pacifidlog Town to say OMG i see Mirage Island Then you use SURF and go east from Pacifidlog till you reach Mirage Island