This is XeroOl's Mod Loader, a mod table reader for OpenITG and NotITG.
To use as a standalone mod reader:
1) Download the XeroOl's Mod Loader repository.
2) Place all of the XeroOl's Mod Loader files in /yoursongname/lua/.
3) In your .sm file, set FGCHANGES to this value: #FGCHANGES:0.000=lua=1.000=0=0=0=====;.
To run with another mod reader:
1) Set up your alternative mod reader of choice.
2) Download the XeroOl's Mod Loader repository.
3) Copy the xero/ folder from this repository into /yoursongname/lua/xero/.
4) In your mod reader's default.xml, add the line <Layer File="xero/default.xml"/>.
5) At the end of your mod reader's Update Command, add the line xero.update_command(true).
After you've set up XeroOl's Mod Loader, you can add mods via xero.add_mod() in mods.lua and put actor definitions in actors.xml.