ReportText is a custom step property, not a static step property.  Thus there is not a "hard coded" function to access it. Here is some code that should work:
	// error checking omitted
void foo(CAObjHandle step)
{
	char *	reportText = NULL;
	VBOOL	exists;	
	
  	TS_PropertyExists(step, NULL, "Step.Result.ReportText", 0, &exists);
  	if (exists)
		{
		TS_PropertyGetValString(step, NULL, "Result.ReportText", 0, &reportText);
		// use reportText here
		}
		
	CA_FreeMemory(reportText);
}