When creating a mod, be it a simple add-on or a fully fledged total conversion, you need to set an entry file so that it can be listed by the Mod Launcher or just started by the game.
This means that you need to create an XML file with name entry.hpc in the root directory for your mod, and its contents will depend on what kind of mod you are creating.
Any valid entry.hpc file will have at least the following attributes:
Keep on reading for specifics on each Mod type:
This page explains the specifics for setting up entry.hpc files for add-on mod type.
A sample mod for a minimal add-on entry can be downloaded here.
This page explains the specifics for setting up entry.hpc files for Stand-alone mod type.
A sample stand-alone mod for a minimal custom map entry can be downloaded here.