aspell SpellCheck icon not visible

I've extracted the aspell.7z and uploaded the aspell folder to the ckeditor/plugins directory. The file permissions are readable by the server.

I edited ckeditor/config.js as below:

CKEDITOR.editorConfig = function( config )
{
   // Define changes to default configuration here. For example:
   // config.language = 'fr';
   // config.uiColor = '#AADC6E';
   config.extraPlugins = 'aspell';
};

When loading an instance I use the following toolbar config:

   $config['toolbar'] =
      array(
         array('Cut','Copy','Paste','PasteText','PasteFromWord','-','Print','FitWindow', 'SpellCheck'),
         array('Image','Table','HorizontalRule','SpecialChar')
   );

The problem is that SpellCheck icon does not appear in the toolbar. No glyph, nothing.

aspell works fine from a command prompt, and the SpellCheck icon works great in FCKEditor 2x.

I'm using CKEditor 3.3.
What could be going on?