The recommended way to get started with TAMods is to download TribesLauncher here. TribesLauncher is an application which automatically keeps TAMods up-to-date, and allows you to run Tribes with TAMods enabled.
Once you've downloaded and installed the TribesLauncher, please close and re-open it (it will not be able to access required system components to inject TAMods when first launched after install).
If the launcher fails to detect your Tribes install path, it will prompt you to locate it. This will generally either be in your Steam folder (something like C:\Program Files (x86)\Steam\SteamApps\Common\Tribes\Binaries\Win32\TribesAscend.exe) or in the HiRez install directory (something like C:\Program Files (x86)\HiRez Studios\HiRezGames\Tribes Ascend\Binaries\Win32\TribesAscend.exe).