It's hard to say which particular crate, however you can find out rather easily by just searching each one, eventually you will find fluffs...
you can't the rum is in the crate with bananas in it
Assuming you're asking about her location before starting Gertrude's Cat quest, Fluffs cannot be found. If you have started said quest, she can be found in the Sawmill (north-east of Varrock) if you ascend the ladders of one of the structures. After the quest, Fluffs will be roaming around Gertrude's House.
It is upstairs the tower in the goblin villlage, in a crate
The plural of fluff is fluffs. As in "the bird fluffs his feathers".
You have to smuggle it out in a crate of bananas. Fill a crate with bananas for Louis, but before talking to him, slip the bottle of rum into the crate. Then have him send it to the mainland; you can find the rum in the back of Wydin's shop in Port Sarim.
You mean the thing that gets pushed around varrock and Falador? that's part of a quest, called "Garden Of Tranquility."
It loads it's self, you just travel to the other side after you have put the rum in the crate, and in one of the crates there should be the rum, if there isn't you have done something wrong ( missed something out ).
When you fill the crate with bananas and rum, make sure you talk to the banana seller again, i didnt do this and i just kept getting bananas, but once i spoke to him and searched the crate again, it gave me the rum.
it fluffs it and makes it chewy
First, you fill the crate with 10 bananas from the banana trees in the back of the man's store. Then, you pick 10 bananas, return to the crate, right click-->Fill crate. This may take you several trips from the tree to the crate if you do not have enough room in your inventory. After you have filled the crate, you then right click the rum-->use rum--->click the crate. It should say: "You stash the rum in the crate" or something similar. If it doesn't work, talk to the banana man again and start the task over again. Then after you succeed in smuggling the rum, go back to the custom officer and ask for a ride back to Port Sarim.
well sometimes it just sorta fluffs out.