The files included in contrib/modified_files/root/ are only the files modified by the mod: it is not a full phpBB installation modified, nor the full mod.

To use it, you have to have a working vanilia phpBB 2.0.21 installation, and to copy over the content of the root/ directory  as well the content of the contrib/modified_files/root/ directory. eg:

- install and configure phpBB 2.0.21 (if not already done),
- COPY root/*.* TO *.*
- COPY contrib/modified_files/root/*.* TO *.*