The '
/xdock' command is one of the tools listed in the MRC scripts that DCX uses, it's not the DLL call.
Make sure the '
dcx_tools.mrc' is loaded, and just
/xdock .., not
dll dcx.dll /xdock ....
Edit: All that being said, having just read through the dcx_tools.mrc, it just 'dll dcx.dll xdock <flags>' anyway..

So if you aren't using dcx_tools.mrc, just remove the
/.