The Manage Devices section should show in the 'iTunes in the Cloud' section on your account on your computer's iTunes e.g.
I think that it will only appear if you have computers and/or devices associated with your account e.g. if you've used iTunes Match, automatic downloads, or re-downloaded past purchases on them - it won't necessarily show computers or devices where your account is logged in (even if you've bought items on them), if you haven't done one of those things on it.
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