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. You will also need an homebrew-enabled Wii. You can find an excellent guide for that at this link.
Method 1 : ReggieDumper
ReggieDumper is a small homebrew app which allows you to dump New Super Mario Bros. Wii's files.
- SD Card (possibly not SDHC) or USB drive with at least 15 MB of free space (for the "Stage" folder only) or 400 MB (to dump the whole disc)
Download ReggieDumper and extract the archive on the root of the SD Card/USB Drive. Overwrite if necessary. The "apps" folder should look like this :
Turn the Wii on and insert New Super Mario Bros. Wii's disc. After that, open the Homebrew Channel and start ReggieDumper. You will be welcomed by this screen :
If you only want to get the files needed for Reggie, press A. To get the entire content of the disc, press B. Let the app do its job, and when it finishes press Home to go back to the Homebrew Channel.
Now, put the SD Card/USB Drive back into your PC. You will find a folder named reggie, with the dumped files inside. You can move it wherever you want.