Can you get the widgets by pressing the fn key and F12 at the same time?
If not, can you see Dashboard.app in your Applications folder?
If you can see the widgets by using Dashboard and you do want to change it from what I assume is the default of F12 then you can do so using the Keyboard Shortcuts. Please note, I don't have a CD on my laptop so can't test if the Eject key is blocked from mapping like this in El Capitan.
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you can see in the screenshot above that Show Dashboard has F12 mapped as its shortcut
you can highlight Show Dashboard to be blue, like the Show notification centre looks above. Then click on the F12 with your pointer, then try pressing your CD Eject key.
If you get an error sound, it can't be done and you'll have to start getting used to using fn and F12. Or if you prefer map another combination such as Option + W instead but you'll have to go through this again as and when you get a new Mac.