If you click through on the promo there is a list of participating hotels. Assuming this property is still listed as participating (it was listed 3 weeks ago when I stayed there) just show the promo details to the manager and if the hotel still will not comply call (on skype) Marriott Guest services and tell them your problem. They will likely call the hotel and work it out for you.

Also beware of a problem I had a checkout. They charged my visa in US dollars when I specifically requested to be charged in RMB. I caught the error/scam before signing the credit card receipt and they assured me that they had corrected the charge to be in RMB but when I received my visa statement it was in fact charged in US Dollars. Being from Hong Kong this may not matter to you but others should be aware. This hotel is very mismanaged as you have seen. Too bad because it is a lovely property and a good value.