Where is the backup folder of TextSecure and Signal? In TextSecure, you can choose to backup your SMS to the SDCard.

It says "Success" but there is no evidence where it does store it on the SDCard.

I cannot find it.
The folder is located just in the base dir of your SDcard:

/sdcard/TextSecureExport


Since Textsecure is now called Signal, there is no extra folder. It is located directly in your SDcard home folder:

/sdcard/SignalPlaintextBackup.xml


Note: sometimes the phone doesen't show the folder on your computer, then you could use a filemanager on your phone (like Ghost Commander) to move the backupped files into another folder that is visible on your computer.