Step 2: Create your ethane molecule

Next we create an ethane molecule. This is almost identical to making ethanol:

Select the C-tool
Click once to make a carbon atom
Click again to make a second carbon atom
Select Edit → Add Hydrogen
And do it again: ... add hydrogen
Click the preoptimize button

You need to add hydrogen twice: the first time one carbon atom was selected. Thus, the 'Add Hydrogen' command added the hydrogen to that selected atom only. By repeating the 'Add Hydrogen' command the hydrogens are added in all possible places.

Alternatively, you can, after placing the second atom, click on that atom to finish using the C-tool. In this case none of the carbon atoms will be selected and you can easily add all the hydrogens at the same time.