If you have a Wii and an SD card, you can cheat online with Mario Kart Wii - regardless of whether it's a backup or the retail copy.

Step 1: You need to install the Homebrew Channel on your Wii. This can be easily accomplished using the Twilight Hack. See the tutorial [url=http://wiibrew.org/wiki/Twilight_Hack#Step_by_Step]here,[/url] and once you've used that, read the tutorial on how to install the [url=http://wiibrew.org/wiki/Homebrew_Channel#Installation]Homebrew Channel.[/url]
Step 2: Now that you've got the Homebrew Channel installed, download [url=http://www.usbgecko.com/files/GeckoOS107b.zip]this[/url] and put the ocarina folder in /hbc in the /apps directory at the root of your SD card.
Step 3: Create a folder while in the root directory of your SD card called "codes"
Step 4: Place [url=http://www.xizer.dreamhosters.com/mkwii/RMCE01.gct]this file[/url] on your SD card in the /codes folder. 
Step 5: Pop in Mario Kart Wii, then launch the Homebrew Channel.
Step 6: Launch Ocarina from the Homebrew Channel, and press A. Ocarina will boot up Mario Kart Wii with online codes.

Every time you wish to play Mario Kart Wii with these codes you must run Ocarina first.

Here are the button configurations to use these codes. You will need a GameCube controller.

Z Button: Blue Shell
Y Button: Fake Item Box
X Button: Three Green Shells
D-Pad Left: Bullet Bill
D-Pad Up: Gold Mushroom
D-Pad Right: POW Block
D-Pad Down: Invincibility Star

[b]ADVANCED OPTIONS (for those who know how to use the code manager bundled with the download that includes ocarina above)[/b]
[url=http://www.xizer.dreamhosters.com/mkwii/rmce.txt]Here is my code file.[/url] It has codes for bananas, red shells, and lightning which aren't included in the above noob tutorial.

This file has all of the codes for online items, but it does not have the hundreds of different button combinations you can use to activate them. You will have to enter those manually. Refer to [url=http://www.xizer.dreamhosters.com/mkwii/buttonactivators.txt]this file[/url] for how to enter those in the code manager.