The slider has an ID number -- if you go to Views > Enable Edit Mode. Then you should see the ID number in the top right of the control. Now look at the actors in the scene. You'll see that some of them have a little "tab" coming out of the input, with a number that corresponds to the ID numbers of the controls.
You can change this number by clicking on the title of the parameter (e.g. the words 'mix amount' of the Video Mixer actor) and an inspector box will appear. Inside you'll see "Ctl ID Link" That's where the ID number is set.
If you take the time to read the chapter "Using Control Panels" starting on page 102 of the Isadora 1.3 Manual (available in the help menu) then I think most of your questions regarding the Control Panel will be answered.
Best Wishes,
Mark