To improve load performance, Mango doesn't let all layers in the legend be turned on and off as you would in a desktop GIS program.
If you have a layer or a group of layers that you would like the end user to be able to turn on or off you will need to put them in a layer group.
Just click on the checkbox next to the layer in the legend to turn it on or off.
See the following section in the documentation for more details.