First we construct an ethene molecule, and preoptimize its geometry:
Select the C-tool by clicking on the button with the 'C'
Select the Double bond mode by selecting Bond → Double
(or use the shortcut: press the '2' - key)
Click somewhere in the drawing area to create a carbon atom
Click again to create a second carbon atom (with a double bond)
Select the select-tool by clicking on the button with the arrow
Click once in empty space so nothing is selected
Select the Edit → Add Hydrogen command
Click the optimizer button
Your ethene molecule should look something like this:
