I would say that it's only worked on the ham/spam that you've fed it. Is it actually catching any spam in the junk folder? You should see results like this:
You can see from this that I haven't fed any spam other than the stuff that comes into Zimbra. I do, however, run the latest developer build which has a better set-up for spam catching & training and I've put DSPAM into that version. I run zmtrainsa if there's any spam that gets into my inbox but that's only a very small number of emails I receive.TS True Positives: 60
TI True Negatives: 59
IM False Positives: 3
SM False Negatives: 19
SC Spam Corpusfed: 0
IC Innocent Corpusfed: 0
TL Training Left: 2438
SR Spam Catch Rate: 75.95%
IR Innocent Catch Rate: 95.16%
OR Overall Rate/Accuracy: 84.40%
I would suggest waiting for the next release of Zimbra (which I believe is coming this week) and install that then follow the instructions for DSPAM in the first post of this thread. That should be all you need to do.
I haven't bothered to install the admin gui for DSPAM yet.