As a bit of background, custom levels are not official add-ons and they're not approved or acknowledged by Nintendo. Custom levels came about because of people hacking the game. Eventually more and more was learned about the Wii and New Super Mario Bros. Wii's inner workings that it became possible to replace the original levels with simple hacked levels. This progressed to creating fully-fledged projects that modify the game to a great extent.
In order to be able to use the level editor, you will need a copy of the game's "Stage" folder, which contains all the original levels and tilesets. There are several ways to obtain the game's files, which are listed below.
From Wii Disc
These methods are fully legal, because they require you to own the game disc.
Method 1 : ReggieDumper
ReggieDumper is a small homebrew app which allows to dump New Super Mario Bros. Wii's files.
- Homebrew Channel
- SD Card (possibly not SDHC) or USB drive with at least 400MB of free space (to dump the whole disc), or 15 MB (for the "Stage" folder only).
Download ReggieDumper and extract the archive content on the root of the SD Card/USB Drive. Overwrite if necessary.
Turn the Wii on and insert the NSMBW disc BEFORE starting ReggieDumper. After that, open up the Homebrew Channel and start the application. What you will see is this.
Now press A if you want to only dump the files needed for Reggie, or press B to get a full disc dump, and wait until it's done. When it finishes, press Home to go back to the HBC.
Now put the SD Card/USB Drive back into your PC. On it will find a folder named "reggie", with the dumped files inside. You can move it wherever you want.