On a recent install of pc-bsd 9.2 RELEASE amd64 (with XFCE 4 desktop) -- in which I was using the XFCE preloaded browser Midori, I couldn't play flash video because it was saying that I needed to install the flash plugin (youtube).
I installed Firefox and Chromium PBIs and they had the same problem.
Then I found this page of the PCBSD wiki:
1. open terminal as user
2. enter this command: flashpluginctl on
3. restart my browser