We couldn’t see the “WordPress SEO by Yoast” under Screen Options on Posts either (although it was ok for Pages). We also confirmed that “Hide” was not checked under the WordPress SEO meta box for Posts.
Here are the steps that worked for us to make it visible:
1. Upgraded the plug-in to Version 1.4.13 (using 3.5.1 of WP)
2. Deactivated and reactivated the SEO plug-in
3. Under the SEO Plugin under Plugins, went to Settings
4. From here, just clicked Reset Default Settings and it magically appeared
