As kuphryn already stated we must fill something
Code:
// NOTE: UNICODE defined
void CDDlg::OnSomeButtonPushed() 
{
   FillDomainsCombo();
}
void CDDlg::OnSelchangeComboDomains() 
{
   FillWorkstationsList();
}
void CDDlg::FillDomainsCombo()
{
   m_comboDomains.ResetContent();

   LPBYTE lpServers  = NULL;
   DWORD dwLen       = (DWORD)-1;
   DWORD dwTotal     = 0;
   DWORD dwPreffered = MAX_PREFERRED_LENGTH;
   DWORD dwRead      = 0;
   DWORD dwType      = SV_TYPE_DOMAIN_ENUM;
   DWORD dwResume    = 0;

   DWORD dwRet  = ::NetServerEnum( NULL,
                                   100,
                                   &lpServers,
                                   dwPreffered, 
                                   &dwRead,
                                   &dwTotal,
                                   dwType,
                                   NULL,
                                   &dwResume ) ;
   if( NERR_Success == dwRet )
   {
      SERVER_INFO_100* pSvr = (SERVER_INFO_100*)(PVOID)lpServers;
      for( DWORD dwIndex = 0; dwIndex < dwRead ; dwIndex ++, pSvr ++ )
      {
         m_comboDomains.AddString( pSvr->sv100_name );
      }
      if( dwRead > 0 )
      {
         m_comboDomains.SetCurSel( 0 );
         FillWorkstationsList();
      }
   }
   else
   {
      // Handle error
   }
}
void CDDlg::FillWorkstationsList()
{
   m_listWorkstations.ResetContent();

   const int nCurSel = m_comboDomains.GetCurSel();
   if( CB_ERR == nCurSel )
   {
      return;
   }
   
   LPBYTE lpServers  = NULL;
   DWORD dwLen       = (DWORD)-1;
   DWORD dwTotal     = 0;
   DWORD dwPreffered = MAX_PREFERRED_LENGTH;
   DWORD dwRead      = 0;
   DWORD dwType      = SV_TYPE_WORKSTATION;
   DWORD dwResume    = 0;

   CString strWorkstation;
   m_comboDomains.GetLBText( nCurSel, strWorkstation );

   DWORD dwRet  = ::NetServerEnum( NULL,
                                   100,
                                   &lpServers,
                                   dwPreffered, 
                                   &dwRead,
                                   &dwTotal,
                                   dwType,
                                   strWorkstation,
                                   &dwResume ) ;
   if( NERR_Success == dwRet )
   {
      SERVER_INFO_100* pSvr = (SERVER_INFO_100*)(PVOID)lpServers;
      for( DWORD dwIndex = 0; dwIndex < dwRead ; dwIndex ++, pSvr ++ )
      {
         m_listWorkstations.AddString( pSvr->sv100_name );
      }
   }
   else
   {
      // Handle error
   }
}