1. Click on add gadget, to open a pop up window hosting blogger default gadgets
2. You should get something similar to the above, those are basic gadgets. You can click more gadgets to see supplementary ones. Just click on any one you want
3. Another window open, with(not in all cases) a preview of the widget. Just farther below the preview are ADD, REMOVE, CANCEL. You know what you want now , hit add.Say you selected 'popular post widget',immediately you click add, all windows will close bringing you back to the layout where you now see 'Popular post widget' already there waiting.