To enter a number in the grid, click on the number on the left, and then click on the empty box you want to place it in.
To enter notes in to the grid, use your mouse to write on the grid. Click on the Pencil tool on the bottom left and then select the number that you want to enter into the box as a note. You can erase notes by clicking on the eraser tool.
Go to the Daily Sudoku puzzle here: http://games.washingtonpost.com/gamedetails/daily-sudoku/#rs=popular