AntiAlt can only be enabled by the absolute owner listed.
To enable AntiAlt, first head to your bot page and make sure you are logged in. If you are correctly logged in, you should see "owner" options below the name and the tags of your bot.
Bot owner panel on the Discords.com bot website
Next, click on "Toggle antialt". You should be redirected to a page saying that antialt was either enabled or disabled.
Antialt toggle success page
Enabling AntiAlt will not in any way change the Discords.com vote counting system if enabled. All votes, even from Alt accounts will count as legitimate, however if an alt account is found to be used, webhooks will not be sent to through the webhook URL (if it was specified).