They may have forgotten to add the object to the library or not want you to use it yet. If you desperately need it you drop these chunks into a custom measure. It will add a SetpointManagerSingleZoneHumidityMinimum to a node that you specify.

Arguments

# the name of the node to add the setpoint manager to
nodeName = OpenStudio::Ruleset::OSArgument.makeStringArgument("nodeName", true)
nodeName.setDisplayName("Existing Node Name")
nodeName.setDescription("Name of node to add 'Humidity Minimum Setpoint Manager' to.")
args << nodeName

Run

# assign the user input node name to a variable
nodeName = runner.getStringArgumentValue("nodeName", user_arguments)

# grab the node that has the specified name
node = model.getNodeByName(nodeName).get

# create a new setpoint manager and assign it to the desired node
newSetpointManager = OpenStudio::Model::SetpointManagerSingleZoneHumidityMinimum.new(model)
newSetpointManager.addToNode(node)