From the web page, went to products, software, then clicked on EyeTV 4 (macOS).
That led me to a purchase page here: https://www.geniatech.eu/product/eyetv-4/
I clicked "Add to Cart", and completed the purchase. Shortly after I received an activation key by email.
Hope that helps.
Note I already own hardware, so I wasn't trying to buy hardware.
Rob.