MODX is used by MOD authors to write instructions on how to install their MOD. downloading any MOD submitted to the 3.0.x MOD database will yield a zip file with (at least) 3 files. the stylesheet and the license are the files that you downloaded. those must be included. the install file (NOT included in the files you downloaded) is the one you open in your web browser. this file will have a .xml extension at the end of it.
regarding the copy action, just do what it says. copy the file that is located on the first line to the phpbb location it specifies on the second line. for example, a MOD may say this:
which means find the file somefile.php
in the root folder of the MOD folder, and them copy it to your phpbb root directory. if the MOD says something like this:
then it means find the file somefile.php
by clicking on the root folder, then the adm folder, than the style folder, and copy it to the adm/style/ folder of you phpbb installation. i know this probably sounds confusing, but im in a hurry (late for work) so hopefully someone can take the time to explain it better.