Android - Where is the backup folder of TextSecure and Signal?

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.