How can I change the word 'Directory'?

Most the texts used in Mosets Tree's front-end can be overrided through Joomla's Language manager.

Alternatively, you can insert the following text:

COM_MTREE_ROOT="Directory"

to the the override file located at this location in your joomla path:

/language/overrides/en-GB.override.ini

Be sure to replace the word 'Directory' with the replacement you want, and replace the override file name's 'en-GB' prefix, if the language you are overriding is not English language.