I was querying Search using SharePoint's object model when I encountered the following error message:

"Your search cannot be completed because this site is not assigned to an indexer. Contact your administrator for more information."

The answer turned out to be quite straight forward. To assign an indexer, launch the SharePoint 3.0 Central Administration console:

SharePoint 3.0 Central Administration > Application Management > SharePoint Web Application Management : Content databases > [select the content database used by your site] > Search Server : Select Windows SharePoint Services search server

That's it!