Set useSecurity from true to false and restart the server
with this you will be able to signup for Jenkin and use that user to login.
To find config.xml
file will be under users home directory. Jenkins will create config file under .Jenkins directory.
you can use below command to find config file
find ~/ -type f -name "config.xml"
this will give path to config.xml file, If you have sudo access you can edit the file.