Im having problems getting this to work.

I tried both the download links, but they go to the same page, and dont allow a nessus client to be installed, just says Nessus version 4.2.0 on both links.
So i installed the old 4.02 version of the client.
Followed the other instructions ok, but when on the program there is no nessus IP to connect to.
So ive put the client i want to scan IP in, then you have to press connect, the edit on a connection manager screen, but this is blank, what IP am i meant to put for the Nessus server? I thought it would of had some default values.

I had a look at the PDF manual and got a challenge code, it said enter this on a web page with your activation code.
So i enter the URL and it says enter challenge code, then enter activation code, but it says the code is invalid?
But earlier it said it was ok when i did it in the CLI using the register command.