1- Grab the machine (transformer) at the left-most down corner of the image.
2- Insert it in the hole near the center (pink portion of the floor). It's between the center and the aquarium.
3- Grab the red monster and place it on the fridege (the machine to the right of where the monster was) It's up of the computer. 
4- Get the battery inside his mouth and place it in the transformer.
5- Now you can fuse with the monsters behind you!To fuse, click on the desired monster. To defuse, simply click on yourself.
6- Fuse with the purple lizard. Now activate the plug on top of the computer. Note that a somewhat kind of wire has come out of a machine.
7- Fuse with the green monster and click in the wire.
8- Now fuse with the spider and click on the wire to obtain a fish catcher.
9- Obviously, use the catcher to catch the fish.
10- Fuse with the new obtained monster and grab the cartdrige in the aquarium.
11- Place the cartdrige in the computer.
12- Click in yourself and you're done!