Use (Keystore / JSON File)/password from MyEtherWallet or windows wallet for login on idex.market and all coin will already on exchange.
Will be another exchange in future?
(Keystore / JSON File)/password from MyEtherWallet is not the same (Keystore / JSON File)/password from IDEX!
You can connect your Myetherwallet with Idex by entering your private key. I assume this is possible with your keystore/Json as well though, but private key will definitely work