Nononono. FOr MOSFETs, there is drain (Vdd) and source (Vss). For NMOS MOSFETs, Vdd should be at a higher voltage than Vss. So Vdd should be connected closer to +12V, and Vss should be connected closer to GND.
For NPN-type BJTs, the collector (Vcc) should at a higher voltage than the emitter (Vee).
Vxx does not mean + or - power supply so much as it means which terminal has to be at a higher voltage and which has to be at a lower voltage.
So depending on "which end" you want to place your transistor, one terminal will be directly connected to GND or +12V, while the other terminal will be connected "closer" to the other power terminal. But since you want to use it as a switch, you want current to flow in series through the load and the transistor, so you cannot connect the both ends directly to the power supply (or else it would be in paralle with the load and not be able to switch the load current). Rather (for the case where the MOSFET is connected on the low-side of the circuit- as close to ground as possible. High side is as close to +V as possible) you connect one terminal directly to ground and since you need the load current to flow through transistor (transistor in series with the load), then you can't connect the other end of the transistor straight to +12V. Instead you must connect it to the negative end of the load (the positive end of the load is the one that goes straight to +12V).
Of course, the terminal which must be at a higher/lower voltage (Vcc, Vee, Vdd, and Vss) becomes reversed when we talk about PMOS and PNP transistors which are "opposite".
Look at Philba's schematic. Both transistors are on the low side (connected closest to ground). So one terminal is connected to ground, while the other terminal is not connected straight to the power supply. Instead it is connected to the end of the load to make a series connection with the load so load current flow can be controlled by the transistor.