I have created my StackOverflow Careers panel, and when I click 'add a stack overflow account with over 200 reputation' the site decides that I have no StackOverflow account at all, although my current one has over 300. I log in to both with the same StackExchange Account with the same email etc so I'm curious as to why this is happening.
Your Careers account wasn't linked properly to your Stack Exchange Account, I fixed it for you.
As reported by Stack Exchange's team there's a workaround for this: just go to your careers profile, switch privacy to private and then back to public. You should now be able to link accounts.