North of the zoo, past the bridge, you'll find a cave. Inside is what seems to be a strange-looking stalagmite. Inspect it and you'll hear a voice speaking into your head. Use telepathy. If you have your Great Grandfather's Diary in your inventory, you'll be able to answer the riddle. After you enter Magicant, you'll no longer need the diary, so give it to the Mysterious Goods Keeper.
Here is the only place you can buy armor. You'll find three shops, marked "7" on their signs (spelling out 777, apparently). Get the best armor from each: Magic Coin, Gold Ring, and Water Pendant (H2O Pendant). The Water Pendant is probably the best pendant, because some enemies burst into flames or explode when defeated, dealing out fire damage, while elemental PK attacks are uncommon until the end of the game (PK Beam isn't so uncommon, but it's not elemental).
North of the castle, you'll find a house with five Flying Men. You can take one as your companion -- you can't control him, but enemies will always attack the Flying Man instead of you. If he dies, you can take another, but remember that whenever one dies, he's gone forever. You can't heal them, either, so they probably won't last long. If you've been fighting all your battles so far, though, you don't really need them.
If you come across Groucho, and you don't have Flying Man with you, always guard. He will eventually say "Hello" and walk off, and one character in your party (just Ninten at this point, but you'll come back later with others) will receive a much larger increase in EXP than you'd gain by fighting. If you're with Flying Man, though, the EXP will go to him, which is useless.
Note: the Japanese version and the prototype English version of the game have a completely different layout for the final area.
Enter the dragon
If you are level 25, you can CHECK the dragon in Magicant to fight it. The dragon is very tough. If you want to do it the "right" way, then Anna needs to know SuprHealing (in case Ninten or Lloyd/Teddy falls). Use Defense Up β and PSI Shield β, and also OffenseUp on Ninten (and Teddy if you have him).
The easy way to defeat it is to buy the Super Bomb at Merrysville Elementary and have Lloyd use it. It will do a great deal of damage to Lloyd, but the dragon will be defeated immediately.
After you defeat the dragon, you will learn the sixth melody.