Once you get into the page for an attraction (or restaurant, or hotel, or whatever), there's a link to 'post a review'. It shows up beside the attraction name on the blue bar directly above 'Views' and 'Reviews'. For the Tiki Room, it looks like this (only formatted better...):
Enchanted Tiki Room Post A Review
Reviews Views
22 105
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Recommended By Average Rating
86% of reviewers 5.7
You need to be logged in to post a review, and I had to log in separately on that page even though I was logged in here - not sure if that's just me or if it's like that for everyone. That 'Post a Review' link is not there if you're not logged in there, though. The link to log in is in the blue bar at the top of the frame (not the top of the page), on the far right. Looks like:
Forum Home · Search · Register · Login
Welcome to the Disney Ratings and Reviews!
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Hope that helps!