Create a folder in your add-ons with the name of the mod.
(assuming you have SVN tortoise already installed).
Right click the folder and choose SVN checkout.
Enter the URL that he gave you at the top of the box that pops up and it will download the mod for you.
Thing with SVN is, some people might have named their folder differently, and so Gmod won't recognize it and still download the same files, only in a different folder.
