Hi. This is perhaps a design mistake on our part. The editor remembers the previous design selection in a Cookie. So to reset you can do one of two things:

1. Delete your browser cache
2. load the browser with design=full at the end of the url, as in http://www.codecogs.com/latex/eqneditor.php?design=full

Shortly there will be a new option on the right of the editor; that allows you to change the design at will.

As you're using the FCKEditor; you should also know that a new version is being released this evening. In this version the latex coding with ... and \[ ... \] has been removed.