Did you not get your download token? It was probably filtered as Spam or Junk. Use this form to re-send it.
But wait! If you expected a token but didn't receive it, it's likely that the email was filtered as Spam. Before you resend anything, add email@example.com to your "white list" of trusted addresses.
Instant Download tokens are sent to the email address associated with the PayPal account you used to pay for the product. Make sure you're looking in the right Inbox.