Move the mouse into the molecule canvas. You will see the symbol of the selected item at the tip of your cursor. It can be placed in the molecule by left-clicking on the desired location.
You can replace any atom in the molecule by placing a new atom on top of it.
You can represent atoms in a molecule with practically any arbitrary symbol instead of standard atomic symbols by creating aliases.
Marvin is chemically intelligent, adds the implicit hydrogens and sets the charge according to valence rules.
In case of metals metallic elements from the 1st and 14-16th groups are added hydrogens, other appear as simple elements as default.