This error usually happens if your host is using PHP 4. The Search For Boss requires PHP 5+ to be able to function.
The system requirements listed on the Search For Boss detail page are:
Windows
* Windows Vista, Windows XP SP2, or Windows Server 2003
* 128 MB RAM (256 MB recommended)
* Dreamweaver CS4, Dreamweaver CS3
Macintosh
* Mac PowerPC or Mac Intel
* MacOS 10.4.x, MacOS 10.5.x
* Dreamweaver CS4, Dreamweaver CS3
Hosting requirements
* PHP 5+
* Linux and Windows hosts supported