To make things easier, I did not use the type "Choice", I created a column named "Test" (type: text) and tried to add it to the Content Query like the tutorial said, but when I tried to import the .webpart, the following error occurred:
"The file you imported is not valid. Verify that the file is a Web Part description file (*.webpart or .dwp) and that it contains well-formed XML."
So if anyone could help me out adding custom columns to the content query webpart, so I can filter them, I'd really appreciate it! Thank you!