It's a question I already know the answer to, but I had to use Google to find the page with the client download.
I still haven't been able to get to find out how to get to that client download page through the website itself. There should be a huge button on the lodestone and on the mog station that says "DOWNLOAD HERE". Either I'm losing my marbles and it's right in front of my eyes and I'm not seeing it, or it's in some obscure place that isn't apparent at all.