In image two, on the right hand side there are two positions marked 'To long / short switch'. If you are using 24" nichrome wire, leave these positions un-connected. Though if you are using 48" nichrome wire, then connect these positions together.
If you are planning on using this power supply for both, then add a switch. With two wires, you can only use one switch.
Long nichrome wire = no resistor (short out with switch), short nichrome wire = resistor (switch remains open).
You don't need an on-off switch, you could just plug in to switch on, though it's better to have one.