Armarium is an interesting word. The dictionary has it as "A place for storing food, clothing or tools". It is also, unaltered from Latin, the place in the vestry where the priest's robes are kept. Ultimately a weapons' store ("arma").
"In armario" = "in(side) the cupboard" (ablative case). "In armarium" = "in(to) the cupboard" (accustative case). Use the ablative case if you're talking about a location. Use the accusative case if you're talking about a destination.