Exchanges does not register your EOS keys to your ERC20 tokens they just sell you.
After you claim your tokens from ICO or buy from exchange ERC20 are transfered to your Ethereum wallet , which has to be converted to EOS tokens after 48 hours of sale ending ,i.e, june 1 2018.
After that period if haven't registered your ERC20 tokens or moved them to the address where you mapped them they will be lost or will be in the mercy of EOS developers
You can follow this article to register
https://steemit.com/eos/@sandwich/how-to-update-the-eos-public-key-mapped-registered-to-your-ethereum-wallet-with-myetherwallet-mew
You can check if your tokens are registered or not in
https://www.myetherwallet.com/#contracts
1.Select Existing Contract -EOS
2.Keys in dropdown menu
3.Enter your ethereum address
4.EOS address registered with Ethereum wallet will be shown
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