By default Insert Components (menu: Assembly | Insert Components) will place components on the Top side of the PCB.
To insert components on the Bottom side, set FAB 3000 View to Backside (check menu: View | Backside). This will allow you to perform all editing directly on the Bottom of the PCB.
Note: When you're done, restore View Backside (uncheck menu: View | Backside).
FYI: The Mirror checkbox flips the component footprints (about Y axis). It do not change the layer which a component is inserted.
Note: You can also change the side (i.e layer) any existing component, by selecting and using Properties... (menu: Edit | Properties...)