Hi Borja,
John is right, that everything is explained in the documents, but it is not so easy (at least for me) to get everything together.
Now let me describe step-by-step how you can toggle the lvar: A32NX_OVHD_APU_MASTER_SW_PB_IS_ON
You do not need a macro, just an offset.
1) Assign a free offset (A000) to your lvar in FSUIPC7.INI:
[LvarOffsets]
1=L:A32NX_OVHD_APU_MASTER_SW_PB_IS_ON=UB0xA000
In: FSUIPC7 Offsets Status.pdf on page 106 it is described that the offsets from A000 to A200 are free for general use. Therefore lets use the first free offset (A000)
Please do not forget to "Add-Ons->WASM->Reload" after you make changes in this section of the PSUIPC7.INI
2) In FSUIPC7 go to "Assignments->Buttons&Switches" and press your joystick button to assign to the APU Master Switch.
Select the checkbox "Select for FS Control"
From the drop-down list of "Control sent when button pressed" select "Offset Byte Togglebits"
Enter in the field Offset: xA000, enter in the field parameter: x01
You see the connection between your button and the lvar is the offset A000
After pressing OK it should work (i just tested it)
Regards
Werner