You go inside the mill and jump onto a ledge. There is a goth girl. Talk to her and trade her for the princess' note. She'll give you a paper with planets on it.
The only "card holder" on Poptropica is for the Cosmic Symbols password on Astro Knights island. Th girl with the card is upstairs in the Mill.
In the fountain. you have to find the password first.
The Cosmic Symbols card lets you access the hidden panel on the town fountain. Pressing the three symbols (and the center Sun) will open the plaque.
You need the Princess's secret message from the maid's room (castle first floor left).
you trad a piece of paper for the password
You get the Cosmic Symbols card from the girl in the mill (second floor). You will need the Secret Message from the Princess (found in the maid's room in the castle). (see related question)
The Cosmic Symbols password card lets you enter the room under the fountain in the village. Pressing the three symbols (and the sun) on the fountain plaque will open the secret door.
Done!
Click on his card in your items bag.
The password card (Cosmic Symbols) is from the girl in the upper room of the mill. You will need to have the Secret Message from the Princess (it's in the maid's room of the castle, first floor left). Pressing the three symbols, and the center sun, will open the panel under the village fountain.
When you obtain the Cosmic Symbols password card, you will notice the similarity to the symbols on the fountain plaque. This is the secret hiding place of the society that preserves Mordred's scientific knowledge. To enter the hidden room, press the Cosmic Symbols on the password card. (the symbols will not work without having the card - see related question)
Take the secret message (from the maid's room in the castle) to the Mill. Up inside the loft is a mysterious girl who will give you the new password for the secret society.(Go to the fountain, press the three symbols and the Sun.)