In the PC version of Maniac Mansion to get past the steel security door on the second floor, the game included a "Nuke 'M Alarms" booklet of codes. This was used as a copy-protection, which was common with floppy disk games of the time to ward off pirates. Since the game is now quite old, many people have since lost their booklets and are unable to complete the game. If you own the game and have lost you booklet, send $2.50 as well as a short note containing your request for a replacement booklet and your home address to:

LucasArts Entertainment Company LLC
P.O. Box 10307
San Rafael, CA 94912
ATTN: Product Support

Note: For more on LucasArts replacement policies, go here. (I would contact LucasArts beforehand because prices have since become cheaper and in some cases they no longer require a proof of purchase)

If you don't mind just having scans of the door codes, then check out these:

I'd also like to point out that thanks to the great folks of the SCUMMVM team, the program now supports Maniac Mansion and you can bypass the copy-protection. Please note that SCUMMVM is made to support legit copies of your old SCUMM games. If you own the game, then SCUMMVM is an option.