Go to tools > options > games and let it detect it. If it doesn't find it, you can add it manually.
If you're using the SI client, you'll just need to backup your original Tribes2.exe and change the name of Tribes2_client.exe to Tribes2.exe for it to be detected.
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If you're using the SI client, you'll just need to backup your original Tribes2.exe and change the name of Tribes2_client.exe to Tribes2.exe for it to be detected.