Support WordPress 3.5?

Yes, the current version of WP-SimpleViewer (v2.3.2) has been tested with (and is compatible with) WordPress 3.5.
When creating a post with a WP-SimpleViewer gallery, click the ‘Add Media’ button, drag-and-drop your images into the ‘Insert Media’ window and, once they have finished uploading, simply close the window and ‘Publish’ your post. (Do not click the ‘Insert into post’ button.)