1 the link is bad ... have you a Download product and which attribute(s) have the download file on them?

2 the Products with Attributes that I did look at have 2 radio buttons with no default button so customers just click Add to Cart ...

It is a good idea with Radio buttons or Dropdowns to add an extra Option Value that is something like:
Pick a color
Pick a size
what do you want?
What media?
etc. etc. to sound good for your site ...

Set to Default YES Display Only YES Sort Order 0 and now the customer must make valid selections ...