I will do my best to be as polite as you. That's version 1.08 you have, and you seem to care about nothing but profits from downloads. In '120428-GG-094526.rar' 120428 means date, 04.28.2012. Version 1.2 was released 05.05.2012.
Try looking here on hongfire for more detailed guide, but in short, you need to get some kind of magic mirror on the southeast of island, then go to sleep in winter house, get attacked by bow-bearing girl(shikigami?), capture her, take her in the party and voila! She will open that door for you.
That may be because not all girls seem to be capturable in v.1.08(and you seem to have that version). I think you need more latter version. Also, to capture senior mermaid(red one, i think) you need a team of humans - little girl in hood, peasant from booty bay island, and witch.