This is a simple guide on how to enable debug on your phpBB3 forum. Enabling debug is a very nice tool that can be used to troubleshoot when installing mods. If something goes wrong and there are errors in your files, debug will give us a hint on how to solve it.

Open config.php
Find:
PHP:
//@define('DEBUG', true);
Replace with:
PHP:
@define('DEBUG', true);
It will output something like this:
PHP:
Time : 0.115s | 17 Queries | GZIP : Off
Debug Extra
Find:
PHP:
//@define('DEBUG_EXTRA', true);
Replace with:
PHP:
@define('DEBUG_EXTRA', true);
Will output memory use and explanation where it is used:
PHP:
Time : 0.028s | 8 Queries | GZIP : On | Memory Usage: 2.25 MiB | Explain
Now debug is enabled you can start troubleshooting.

Note! config.php contains your database info, make sure you don't overwrite anything important.