I just edited the wp-config.php file and added that line without problems
/* That's all, stop editing! Happy blogging. */
Did you add it in a different place?
Apart from that, I also edited the main php.ini file and set the new memory_limit value
; Maximum amount of memory a script may consume (128MB)
memory_limit = 256M
Finally, I restarted the services and checked the information with a PHP info page
How did you check the WordPress' memory limit value?