The article in the User's Knowledge Base shows it positioned immediately 
  before the call to:
bo.PostProcess()
http://www.amibroker.org/userkb/2008/03/16/amibroker-custom-backtester-interface-2/
Copy/paste 
  the name of the metric from the documentation to make sure that you've got it 
  right (scroll to bottom):
http://www.amibroker.com/guide/a_custombacktest.html
Mike
--- 
  In amibroker@xxxxxxxxxps.com, 
  "Markus Witzler" <funnybiz@xx.> wrote:
>
> 
  Hello,
> 
> could someone tell me where to put those two lines in 
  CBT low-level mode:
> stat = bo.GetPerformanceStats(0); 
> 
  
> Maxx_dd = stat.GetValue("Maxsystemdrawdown");
> 
  
> I guess I´ve tried everything, but max_dd turns out to be always zero 
  when I add it as bo.addcustommetric("Max_DD",max_dd);.
> 
  
> Thanks
> 
> Markus
>