(1) If you are trying to create a post on the WordPress.com home page here http://wordpress.com/#!/post/ then forget that please. Instead create posts only on your own dashboard here > Dashboard > Posts > All Posts > Add New
http://NAME_OF_BLOG.wordpress.com/wp-admin/post-new.php

(2) Are you using the secure https:// login?
http://en.support.wordpress.com/https/ If not then try using it.

(3) Exactly which browser and version of it are you using. If you aren’t sure click here to find out > http://whatsmyuseragent.com/

Note: If you are using an IE9 browser in compatibility mode that makes it render as IE7 and IE7 is not supported. To disable see > http://www.sevenforums.com/tutorials/1196-internet-explorer-compatibility-view-turn-off.html

a. Try using the browser with all extensions or add-ons disabled

b. If applicable try updating the browser to the latest stable version http://browsehappy.com/