It’s an svn, you need to get a program like TortoiseSVN to download it- http://tortoisesvn.tigris.org/
I can’t say it’s the easiest way but it’s the best way for any gmod addons.
To learn how check out this video on how to use it and which one to download- http://www.youtube.com/watch?v=MYtIMmIM09Y
In the video they show how to download with similar links as the one you shared with me, all you do is apply the same things to your link instead as the ones in the video
You should also think about downloading the svn versions of PHX and Wire Mod that are show in the video.
By the way, Next time post here- http://www.facepunch.com/forumdisplay.php?f=16