Source: http://forums.ubi.com/groupee/forums/a/tpc/f/9991033023/m/4511046244/p/1
Okay people,
After ( too ) much tries, we ( me and pok2, we did the online test! ) can finally say to you: "WE HAVE RAYMAN ARENA WORKING ONLINE!!!" lol. Yes, it's true. Via Hamachi.
What you need to do is:
1. Installing:
Download & Install Hamachi:
http://files.hamachi.cc/priv/HamachiSetup-1.0.0.56-en.exe
2. Connecting:
Open the Hamachi program, and click teh connect button.
If you're the SESSION MAKER, click the 'networking menu' button ( sorry, you'll have to search for this one, but there are only 2 buttons, so...
) and click Create new network. Make a Name and a password for the network, then let other people join with the following instructions.
If you're the SESSION JOINER, just click the 'networking menu' button, and click 'Join existing network'. Type the name and password you recieved from the session maker, and click OK. Now you can see all other people who joined the network.
3. A VERY IMPORTANT STEP:
Go to Network connections in your windows configuration panel. ( If you don't know how, Start-> Run -> ncpa.cpl
)
Close your internet connection ( not hamachi, keep that running, but just close your internet connection. ), then run Rayman Arena.
4. Rayman Arena Trick – IMPORTANT!
First I'm gonna give instructions for the SESSION MAKER:
Now Rayman Arena is started, open the LAN game.
Type your name, and click confirm ( or whatever ).
Click Create. You won't see any games open. It's very normal. Type your session name. Choose your character & map. Click confirm.
DO NOT CLICK CREATE YET!!! IT'S VERY IMPORTANT!
Now go back to your desktop, leaving rayman arena on. ( Alt+Tab is your friend!
)
Then Re-open your internet connection. Be sure to wait until Hamachi is reconnected.
Now open your Rayman Arena, and click Create. Server should be successfully created!!!
Now wait for some session joiner(s) to join, and click start, ( or whatever it is ), and prepare for a very smooth game!
If you're a SESSION JOINER and the SESSION MAKER already created the server, open LAN game menu, type your name, click confirm. You’ll not see any games running. Normal.
Now go back to your desktop, leaving rayman arena on. ( Alt+Tab is your friend!
)
Then Re-open your internet connection. Be sure to wait until Hamachi is reconnected.
Now open Rayman Arena ( DO NOT CLICK REFRESH! ), and what do you see? Tadaaa! There’s your server you should join! Select it and click join, and prepare for a very smooth game!
That's it!
Also, if you're using firewalls, be sure to disable them for the Hamachi connection
Pok2 and I had a very smooth game. He was Razorwife and I was rayman. He chose First Ruins as map. And I beated him! He arrived 2 seconds after me. The game was as I said, very smooth. No problems at all!
Also, I'll edit this post when pok2 finds a way to make this thing less difficult.
And... who wants to play? 
Please notice, this method is only for people who have Rayman Arena, not Rayman M. Also, each time you want to play, you'll have to do the steps starting from step 3!
Have fun!
It doesn't work for Rayman M, because it hasn't got any LAN option.
Also make sure in your Rayman Arena options your connection is set to: Local Area Network.
Droolie.
