The "start it playing on the phone" trick appears to no longer work. It no longer loads more songs "as you listen", if it ever did. I could have sworn I had tested this, but I just noticed I've been listening to the same 10 or so songs on repeat all day. I tested it with spotify open and watched it repeat back to the top instead of loading more songs. It loaded more songs after plugging my headphones into my phone.
Until they release it for the desktop, simply start playing it on the mobile device. Next, click on "Devices Available" and select your desktop instead of the mobile device. You can then control it as normal from the desktop app.
Re: How do I find the "Your Daily Mix " on a Laptop/PC?