Hmm, that's strange.
Is the core service (the one in which your Fetch set top box is attached to) linked to your My Account? Is the Fetch service linked to a mobile or broadband account? You'd need to ensure that you're clicking on the relevant product and accessing the drop down menu from there.
If you're still having no luck, you're best bet is live chat → www.optus.com.au/livechat/pbb. This is a really simple request; they'll have no issue pulling your activation code from their end. If you're still having no luck, message us back here and we'll gladly help out 🙂