The magnet link is for torrenting- it generally will download faster that way but you’ll need a client such as bittorrent or utorrent. as for the direct download:
its the giant green button that says download…
this is the direct link
http://downloads.petrockblock.com/images/retropie-v3.0beta2-rpi2.img.gz

its possible if you have a javascript blocker that it might not show up. But as of right now when I navigate to this page https://www.petrockblock.com/retropie/retropie-downloads/retropie-project-sd-card-image-for-raspberry-pi-2-version-3-0-beta-2/ the big green button is there.