If both coins have the same procedure for generating the address from a private key, then the same private key can be used to access coins in both block chains.
Accessing the coins is straightforward. You can't send coins from one block chain to another. You sent the WC to a WC address that happens to be the same as a WDC address. mcxNow has the WDC private key in their wallet, If the addresses are compatible, all mcxNow has to do is to import that same private key into a WC wallet, and then they can access your WC.
^ This.
MCXNow does not have your WC -- you sent it to a WC address that (most likely) no one currently has immediate access to. However, if the WC address that you sent it to and the WDC address that your MCXNow account is linked to share the same private key, then you may recover your coins if MCXNow accesses that WC address and sends it back to you. Alternatively, they can cut off the middleman and just tell you that private key, but relinquishing one of their private keys would be going beyond what they are responsible for business-wise, which, based on your account of the story, is absolutely nothing.
Truth be told, I don't expect you to recover your coins if Cryptsy has already sent them to the new address. However, since Cryptsy is painfully slow at processing withdrawals, then assuming that your withdrawal is still pending, I would highly suggest asking Cryptsy support to cancel it if possible. That might be your best shot at recovering your funds.
