I had to install it manually.
I am not good at debian, I just download and estract the btsync binary file in Public share, then I use putty or Win SCP to run it:
chmod +x /shares/Public/btsync
/shares/Public/./btsync --webui.listen 0.0.0.0:8888
that’s all.
over at btsync forum told me to play around with the config file to set a different folder rescan so as to allow the EX2 to sleep, but I did not have time to do it, and personally I expected them to look into this issue sonce it affects many NASes.