(Note: I followed the instructions 100% as is what they stated. So it's nothing on my end. Yes, all the steps.)
It's asking for a CD key. Also, Tunngle does nothing but throw ads at me constantly asking for money.
How do I play multiplayer on this?
Quoting the OP where it stated the MP part.
HOW TO PLAY AVP2 ONLINE IN 2017+
For those that still want to play this awesome game! then simply download the (TUNNGLE APPLICATION) @ https://www.tunngle.net/en/download
This doesn't just help you play AvP2 online! it also lets you host dedicated servers via the LAN/Option which is bypassed and access worldwide.
~ HAPPY HUNTING))
...... Okay so no CD key, and Tunngle is just ads... Wanna update and try again?