You would not usually access this page directly. It should be loaded by one of the many WYSIWYG editors that we support (i.e. CKEditor), which will launch the page and then when you press 'Copy to Document' it copies it back to editor.

For further instructions see http://www.codecogs.com/latex/integration/ckeditor_v4/install.php

or if you really do just need a standalone editor, then use this: http://www.codecogs.com/latex/eqneditor.php