you can try these
1) find out your PC name like mine mine is John-PC and enter it and enable your PC to accept don't know if its different i never used a mac
2) kick the computer ok don't kick it but restart it
3) you may need to set up a server for it to conect
Thank you for the tip policejohn99.
I JUST FOUND THE SOLUTION and it was on my Mac settings !!!
I am using Mac OS X Lion, it may be the same solution for other OSX version. You have to go to Preferences - Sharing - then click the Checkbox "Remote Login" ("Session à distance" in French). Then is suddenly and mysteriously started to work.