The following is a pretty straight-forward guide on how to boot New Super Mario Bros on PAL/NTSC systems with firmwares 4.2 and below without the need for any custom IOSes. This guide has been around for awhile now and was first released at EurAsia.nu by modrobert. It was re-written here to be more noob-friendly.(Link)

OJECTIVE: To be able to play the New Super Mario Bros on a chipped system without a hitch.



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WHAT'S REQUIRED FOR THIS TO WORK:

1. Chipped system e.g. d2lite, DriveKey, D2CKey, WiiKey 2, d2pro etc.
2. Homebrew with at least HBC installed
3. Gecko (dowload link provided)
4. NSMB cheat code (download link provided)
5. SD Card
6. Un-patched/virgin release of NSMB

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SD CARD SET-UP:


The files you need can be obtained here. Choose the appropriate video standard (PAL or NTSC) for your Wii.

These are the two folders that you will need.



These are what's contained in those two folders.

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Once you've downloaded the files extract those two folders onto the root directory of your SD card. Please note that if you have a pre-existing apps folder in your SD card you only need to copy what's contained inside the Gecko 1.92b folder (see image above).

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Wii SET-UP:

1. Take your SD card and insert it in the Wii's internal SD card slot.
2. Fire up HBC and launch

3. Enable cheat in the Config Options.
4. Insert the game disc.
5. Select Launch Game, the first option.

Gecko will use the cheat code present in the SD card to load NSMB with no crashes.

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CREDITS:
*modrobert for converting cheat code for Gecko OS
*Nuke for Gecko OS application
*Team Twiizers for the Homebrew Channel
*Folks over at GBAtemp for the discorvery and development of the cheat file

I'm glad it worked for you!

Folder download does not work. However, you can still download each individual file.

Determine what version of NSMB you have (PAL or NTSC). Then make sure you get the appropriate .gct file to go in the codes folder.

I've failed to mention that this is to play NSMB from disc using a modchip. I will edit the guide now. Also, you need the un-patched disc/release.