[Help]Not able to detect ftf files using flashtool

I have rooted and unlocked my bootloader from Sony official website and also flashed a ROM. But I wish to go back to my stock ROM but flashtool doesn't detect ftf files i Downloaded.
I had installed the drivers and disabled signature verfifcation in windows 10.
Should I use fastboot and flash system files .sin file after extracting the ftf files???????
Please give a step by step guide.
Had dsabled .git sync:silly:
flash tool log:
19/032/2016 08:42:44 - INFO - Flashtool Version 0.9.22.3 built on 08-06-2016 22:30:00
19/042/2016 08:42:44 - INFO - Executing search strategies to find proxy selector
19/042/2016 08:42:44 - INFO - No proxy found for IE. Trying next one
19/042/2016 08:42:44 - INFO - Strategy firefox failed trying next one : No Firefox installation found
19/042/2016 08:42:44 - INFO - No proxy found for java. Trying next one
19/042/2016 08:42:44 - INFO - Loading devices database
19/042/2016 08:42:44 - INFO - Loaded 0 devices
19/042/2016 08:42:44 - INFO - Starting phone detection
19/042/2016 08:42:49 - INFO - Device connected with USB debugging on
19/042/2016 08:42:49 - ERROR - No device is registered in Flashtool.
19/042/2016 08:42:49 - ERROR - You can only flash devices.