Note that when using any Transformer, the 3-dotted side means higher voltage and the 1-dotted sides mean lower voltage. You can change the direction the 3-dotted side is facing by right-clicking a 1-dotted face with a Wrench.
To convert HV to MV place the input cable (HV) next to the 3-dotted side. They should connect, and run your output cable (MV) next to any 1-dotted side connecting to your machines, or energy storage units.
To convert MV to HV you must apply a redstone signal to the transformer or change the setting in the GUI, and place the input cables next to any 1-dotted side, and the output cable next to the 3-dotted side.