Finally got this to work
So, I tried using my mileage plus account number to use this offer, and it kept telling me invalid account number. I called Hilton and United, and neither were able to help. Hilton did say that since I was an existing member, I need to create a new account, and then call back to get the two merged.
Eventually, I figured out that the problem was that I was using lower case letters in my mileage plus account number. When I put capital letters in, it worked (Really?). Once I got the new account number (using a different email), I called and had the two merged. Gold status applied!
Hope this helps.