Q:

Trouble using a generic 4-20mA to 0-5V board to connect transmitter to Arduino

I'm doing a big project, where I will connect a lot of sensors to my Arduino Mega 2560. Eagerly, I bought boards of this type, but I can't get them to give me any voltage output! I've connected them with +24V, and my 2 wire 4-20mA sensor, a pressure transmitter, to the other end. 
There's no documentation to be found about the board, despite it looking like a popular product. 
I have Vout measured with a voltmeter, it says 0.13. It's no different than the other cards, where there is no sensor attached at all.
I was expecting there to be +24 between I+ and I-, but there is nothing. It's 100ohm resistance between them internally in the card.
I have 16 of them, and they all behave the same, so there must be somehing I'm not doing right Could it be that they are made for 3-wire sensors? How can I wire it differently in order to make it work? Here's a photo of the card:

A:

Your loop powered (two wire) transmitter + sensor has two leads, call them S+ and S-. 
You need to connect one end of your two-wire sensor to the +24V (S+) and the other side (S-) to the + current input. No connection to the - current input (it's common with the power supply ground). 
The two-wire sensor needs a power supply and you are not giving it any. 

You should adjust the pots to ensure that  you don't get more than 5V out of the outputs before connecting it to the Arduino. It would also be a good idea to clamp the output in case there is a fault with the sensor but that's outside the scope of this answer. 

