Emailing MR rewards works VERY well. Just be concise with issues. Sometimes people go on and on and the real message is lost. Log in to your MR. Click on your account. Then contact MR. There will be an option to email. Their email address is email@example.com. I think they like the method I first described because the portal has fields for specific guest and stay info.
Sorry about your experience; I've seldom experienced this at any Marriott facility. However, you might also try completing one of the in-room commentaries and leave it with the front desk. You may, or may not, receive any further contact about your complaint, and if not, you might try contacting customer service at Marriott headquarters. If you think this was perhaps an isolated incident, try returning to the hotel on your next stay in the same area; if you experience the same, move on to next available location, and report the same to Marriott. A continual, or repeated, bad experience is likely to get their attention more than just one incident.
tker is absolutely right. Email is the best method of communication for both parties. It keeps a written record of the conversation and allows you to organize and clearly state your issues. I am disappointed you had a bad stay, I imagine a well thought out, sincere complaint to customer care is your best bet for a positive resolution.