Soooo...
How do I detect it? Remove it?
You will find a video here explaining that: How To Remove MACDefender Malware Software [Video How-To].
If you get a warning that your Mac is infected with malware while surfing the Internet, you've detected it. Don't believe it! Don't click on any part of it. Ignore it.
Don't download it, don't install it. Don't give it permission to install.
If you installed it, here's how to remove it simply