Do you have a wad installer (such as multi mod manager) installed? If not:
1. Download multi mod manager (search on google)
2. Place it in the apps folder of your SD card.
3. It is now ready to be started from the homebrew channel.
Now, to get the game:
1. Make sure that you own the original (if not, this becomes illegal)
2. Download the game's .wad file (just google the game name with wad after it)
3. Put the wad file in the wad folder of your SD card (or anywhere else, it doesn't matter)
4. Start MMM and click on "install wad". It will show the contents of the wad folder on your SD card by default.
5. Locate the file and install.
6. Start your game from the wii menu!
Hope this helped, this method is the same for WiiWare games too.
Also, I'm guessing that your wii is already softmodded - but you should still give as much info as you can when asking a question!