The unique visitors will show up in the following time:
But can't get unique visitors in specified timestamp range, why?
Do you mind translating what it says? That would be helpful. You can also change the language to English temporarily by clicking on your username in top right.
As you suggested, here's the detail.
I can get the unique visitors at the bottom right;
But I can't get unique visitors when specify the date range.
Thx! Please have a look here: http://piwik.org/faq/how-to/faq_113/
You need to enable it specifically if you want to use it.