theres a lot of bugs in the game now, and even i have three cats, a teddybear and cant find the yellow key, but ive done all the quests (including sushicat) but the wooly flight thing....
There's a bug, I think that if you talk to the Elephant near the yellow door and open the door just after, the quest won't complete (i had the same problem....
when you complete the yellow key quest, the quest you change into finding the green key and so on, until you complete the game
Balloon 1: area 15, center, under platform Balloon 2: area 27, bottom left balloon 3: area 28, high platform Balloon 4: area 36, next to bottom left door Balloon 5: area 7, middle of the screen Balloon 6: area 13, left of left door Balloon 7: area 5, right of door Balloon 8: area 31, between two doors Balloon 9: area 14, where the tanks are Balloon 10: area 10, you have to come down from the top door