undefined symbol

Iam getting the following errors when i try to compile my application. while i have include files

#include "dModt.h"
#include "dModt_Demodulation.h"

but i get this errors.

  error: Undefined symbol '_ModtGetRecoveredBitStream@12' referenced in "c:\Proj\cvibuild.SIGNAL\Debug\Capture.obj".  
  error: Undefined symbol '_ModtMSKDemodulate@72' referenced in "c:\Proj\cvibuild.SIGNAL\Debug\Capture.obj".  

what could be the reason?