STPM10 using Arduino SPI communication
The STPM10 specification describes a SPI bus unlike any I have seen.  Apparently the STPM10 "SYN" line controls the direction of the "SDA" line.  As far as I know, there is no "SYN" line in a SPI bus.  As such I do not think a normal Arduino SPI library will talk to an STPM10.   

A normal single port (MOSI and MISO combined) SPI slave would automatically switch between reading and writing bits.  When interfacing with a two port master (such as an Arduino) a resistor is used between the MISO and MOSI nets.

This is the best image I could find.  Ignore the chip names.  And substitute MOSI for SDO and MISO for SDI.  Note there is no "SYN" line in this SPI bus.



I'm showing this diagram as it appears the STPM10 is using a single line for both input and output data.

Because of the missing "SYN" line I do not think you can even use the SPI hardware in the Arduino's Atmel processor.  I think your only option is to create a SPI driver from scratch only using software.

I see where STMicroelectronics offers an evaluation software package that talks to the STPM10.  Guessing, it looks like they wrote it in National Instruments Lab View.  If you could get the source code for that it may provide hints on writing your own STPM10 driver.