First, rename your file so the extension is .bin instead of .key. Uwizard doesn't include .key files in its file dialog.
In Uwizard, click on "Open WiiU Game", and navigate to the .wud file.
Then click the Import button in the Title Key section, and navigate to your .bin file.
After you select your file and click Open, click in the title key box. Press the home key on your keyboard, then use the right arrow key to count the characters.
If you have 64 or 66 characters, your key file is probably a txt file. Rename the extension to .txt and try again.
If you don't have exactly 32 characters, your key is wrong.
Also, when you click Extract Game Files, the DiscU output will tell you if your key is incorrect. (Move the "Error Extracting Files" window if you can't see the DiscU messages.)
If you need more help, you'll have to give us the actual DiscU messages. Messages from frontend GUI (Uwizard, in this case) aren't always useful.