User can write reviews when logged into the site through either Facebook Instant Personalization or a Rotten Tomatoes native account.
If you are logged in via Facebook, you will see Your Name at the upper right on each page, with a toggle button and either "Social On" or "Social Off." You'll be able to go to any movie's details page and rate and/or review the movie using the stars and text box directly under the Tomatometer. If you have toggled "Social On," your reviews and activity on the site will post to Facebook. You can also check the Twitter box next to the review section to post to your Twitter account.
If you are logged in through an RT native account, you'll see "Welcome, [Your Name]" at the upper right on each page, with dropdown options Profile, Community, Messages, Account and Logout. You can rate and/or review movies on their details pages, underneath the poster image. You can see all of your ratings by clicking on Profile in the dropdown at the upper right. If you are logged in via Facebook and want to use an RT native account, or vice versa, just log out via the dropdown at the upper right and then log in to the correct account.