One of my screenplays was recently posted by the producer. How do I get my name under the writing credits?

If this it the title you're referring to (http://www.imdb.com/title/tt3300604/c...), you should wait a couple of days, until the page is completely updated, and see if your name has already been added. Newly added titles take up to a couple of days before all contributed data is actually showed on the site.

If at that time your name is still not there, you should use the Edit page link at the bottom of the movie title page find the line for Writers and choose "Add 1 credit" in the box next to it, then click Continue at the bottom. You can then follow the instructions to add your name to the list of writers.