Unfortunately Jaksta for Mac cannot capture from BBC iPlayer at the moment. We are looking into fixing this in an update. Sorry I can't be of more help here.
Certain sites often change the way they stream video, so we have to keep upgrading Jaksta to support the latest streaming methods. This is what has happened with BBC iPlayer.
I recently updated to the latest version of Jaksta on Mac. I'm using iPlayer, on Chome with a Hola plugin, allowing me to access iPlayer from overseas. iPlayer plays fine in Chrome but Jaksta is not detecting it.