Just trying to interpret here, but If I understand it correctly, you need the "Eternal Crosier".
First off, you probably have entered a tower north of Nelusco, defeated the demon there and got the 4th key. There should be a maze there, where you find Dinges and talk to him. He asks you to find the Eternal Crosier.
When in Nelusco, go eastwards, and there should be a place in the forest where you have to open 8 chests in the right order -
2 1
7 4
8 3
5 6
9
This should give you the Eternal Crosier, with which you can enter the gate at the beach.
I hope this helps. If not, I suggest that you run the text of the walkthrough through Google Translate.
I got it. The gate refuses to open. In fact I used my iphone to determine the proper combination to unlike the chests in proper series. I got the eternal crosier and when I went to the beach...the gate remains closed and still prompt me saying that there is a mechaninsim somewhere.
