How to get the Reggie! Level Editor

First, you will need to download the Reggie! Level Editor, Riivolution, and the Reggie! Dumper, as well as the New Super Mario Bros Wii retail disc.

Level Editor and Dumper:
http://rvlution.net/reggie/

Riivolution:
http://www.rvlution.net/riiv/Riivolution

After, put the Dumper folder and Riivolution folder under the apps folder in your SD card. Open up the Dumper (have New Super Mario Bros Wii in the Wii), and select to dump a bare minimum.

Bring your SD card back up to your computer, and load up the Reggie editor. It will ask you where your "stage" file is. It is in SD/reggie/stage, now you can make levels. Place your levels directly in a /nsmb folder on your SD card. For example, if you wanted to replace the level 1-1, the file on your SD card should be SD/nsmb/01-01.arc

You can't play the levels through Riivolution yet. Go to here: http://www.rvlution.net/riiv/Patch_Templates and see where it says "New Super Mario Bros Wii". Find the "download" link and right click save as. It goes right into the "riivolution" folder (which should be at the root of your SD card)