Okay this one is a little bit tricky....
First of all you have to use the PC in Ed's Room (They guy with the stamp book) to play Maniac Mansion, which you can find with Bernard going upstairs. When the Game starts, pick Dave, Razor and Syd. (it's important to use those characters, I'll explain at the end why)
When the game starts go to left with Dave to the front door of the house. Pull the mat to find a key underneath, pick it up and use it to open the door. As you enter you see a door right next to the clock, open it. You are in the kitchen now, as you go a little further to the right you will meet Edna and get captured by her at the fridge (this is necessary).
Once you are in the Dungeon switch to Razor (with the New Kid command), go to the front door again and enter the kitchen (Edna is gone now) go further to the right, open the door and you will enter a dining room ignore the meat on the table and go to the right again to find another door. You are now in the storage room the only thing you need to pick up here are the fruit drinks (they are in the bottom left of the shelf). Leave the room the way you came from. Back to the entrance where the clock was. Now go up the stairs and to the room to the left. You will find an artist room. Pick up the bowl of wax fruit. Leave that room again and go through the security door in the middle. You will be in a hallway with a statue to the left. Go right to the stairs and further up. You'll now meet green tentacle which wants to be fed, give him the bowl of wax fruit and afterwards the fruit drinks and he will let you pass. Go to the left and up the stairs again, enter the first door you see and wait in there.
ALSO IMPORTANT NOTE: If you are not familiar with Maniac Mansion and all the rooms you will probably take a while, if you ever get a cutscene where Ed says he is getting hungry, stay away from the stairs and the kitchen for a while.
Okay now for the tricky part!
Switch to Syd outside and let him go to the front door outside. In the middle of the left door is a doorbell, ring it! Change back to Razor again and wait for about 10 seconds before you leave the room (otherwise Ed will catch you) enter the third door from the left, and you will find Ed's hamster, take it quickly and go back to the room where you were hiding: (You will see a cutscene in between where Ed will answer the door and tell Syd to go). Also it's important that you wait at least a minute in your hiding room when you saw that cutscene! otherwise there is a big chance Ed will catch you on his way upstairs. Now the last few steps after you were waiting in that room, go back down with Razor all the way to the kitchen. Open the microwave, use the hamster, close the microwave and turn it on. "DING" achievement unlocked!
Side Note: Those characters were chosen because of their traits, for example Bernard can't get past Green tentacle because he is scared of him and can't give him any food. Razor is also one of the characters that can kill the hamster, other characters refuse to do this action. You are also able to get by Green Tentacle to feed him the Cheese, Lettuce, Rotten turkey, rotten ham, ketchup, canned food and tentacle chow (which all can be found in the fridge, dining room and storage) and then give him the fruit drinks however this takes much longer and Ed will probably get hungry. I'm writing this to give you another chance if razor get's captured by any chance. Because Syd is also able to microwave the hamster.