Marvin has a number of options for the display of implicit and explicit hydrogens.
MarvinSketcher adds automatically the hydrogens to the structure. Generally, these are implicit hydrogens and are displayed based on the options set in the View menu.
To view all hydrogens explicitly displayed as atoms with bonds to neighbors, chose Structure > Add > Add Explicit Hydrogens. The Structure > Remove > Remove Explicit Hydrogens will return to the previous display mode.
To view implicit hydrogens by symbol, use the View > Implicit Hydrogens menu group. This option is disabled in Spacefill and Ball & Stick display modes.