This option contains a URL pointing to the installation executable of ieSpell.
By default it points to the list of the ieSpell mirrors:
FCKConfig.IeSpellDownloadUrl = 'http://www.iespell.com/download.php' ;
As you may see the default address isn't pointing to the installation executable of ieSpell. The reason is that mirrors on the ieSpell site are constantly changing, so pointing to one of them would be quite unstable. We have contacted ieSpell, asking them for a better URL to point to, but we had no response. So, we recommend downloading the latest version from their site (http://www.iespell.com/) and placing it in your server, changing the "IeSpellDownloadUrl" setting to the new location.