In the PCB editor when you are moving or placing a component, just hit the hotkey "L" and the component is moved to the opposite side of the board.
In the Library Editor, when you are placing the pads initially, hit the hotkey "Tab" for the first pad, and select the bottom layer. All of the pads you place from then on will be on the bottom layer.
Another way to move things to another layer in the Library Editor is to use "Find Similar Objects". Once the matching objects are found, just change the layer in the Library Inspector Panel - that does a global change.