Only modules which implement the ISearchable interface participate in DotNetNuke's search capabilities. If you are asking how you can implement ISearchable in modules which you are developing, here is the Wiki reference:
 
http://www.dotnetnuke.com/Resources/W...

If you are asking which "core" modules implement ISearchable and which do not, you can sometimes find out which interfaces are implemented in the module project's release notes or other documentation, in the on-line manual on this website (http://www.dotnetnuke.com/Resources/S...), or by examining the module's registration information using the Host --> Extensions page on your site. Third party module developers and vendors should clearly state whether ISearchable is supported or not.

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner

I did implement the ISearchable interface but nothing happened :S still cannot search in my module :(
Any thoughts ?