You should take a look at mongod.conf file.
Default working directory should be "/data/db" - if is now exist - just create is. No need to chmod 777 if you create the directory and run mongo as root user.
If you want to start mongod with other working directory - modify mongod.conf file or start mongod with --dbpath /pathtootheremptyfolder/
Please let me know if you are OK.