Getting the $5 bonus was a little unclear to me, but pretty simple. I already have a DD account, their app, and a card with some credit on it.
First issue was getting Masterpass set up. I pulled an inactive Citi Mastercard that looked good, but it was rejected from setup via masterpass.com. The message was crazy, because as I read it, it told me only Citi Mastercards were eligible, and that if I had a Citi Visa or Amex I could sign up with that instead. Either I or that error page missed some words, maybe my flavor of Mastercard was not eligible. So then I thought to try my Visa daily spender (Alliant) and Masterpass took it.
Getting a new card via the desktop seemed to have a $5 minimum, but via the app only $2. So I signed up there, and it immediately reflected the bonus amount. Merged the cards, DD app crashed and restarted itself, and it had merged my balances onto one card correctly.