I have installed the plugin without issues but when I have the WP to Twitter plugin enabled the 'Add Media' button when adding/editing a post doesn't seem to work.
I have looked at the code and there appears to be a JavaScript error caused by the plugin which is stopping the add media popup from displaying. From looking at the JavaScript console I can see this error:
Uncaught TypeError: Object [object Object] has no method 'charCount' post-new.php:39
It looks like the JQuery charCount library isn't being loaded before it is used but I can't seem to see how to fix it.
I am using the latest version of WordPress (3.8.1) and the latest version of WP to Twitter (2.8.3).