It's under the black cover.  I think it's a Wire strain gauge. (trådtöjningsgivare)

I haven't bought any flow sensor yet. 

I have seen there is different kind of  output signal to choose, like CAN, Digital TTL pulse (0-5V) and Analog voltage (0-10V) .

Any you recommend to use ? 

Hi, 

No amplifier is necessary. I cannot see where the load cell is, but in any case just measure the distance between the load and the brake center for the load arm length. 

For the fuel flow meter, is it a type where it outputs a certain number of pulses per sec depending on the flow. This is not supported directly today, but is an easy software addition. A few guys have asked for this lately so I will see if I can add this in the next release. In the meantime you can use a frequency to voltage converter and use an aux port.