jcl's guide works for me.

I got similar issue that "Error 053: UK100 unavailable at this time" when trading Z12.
My broker's corresponding symbol is called ".UK100Cash", so I added a line in the assetFix function of Zorro EA(Zorro.mq4).

if(Asset == "UK100") return ".UK100Cash";

Then I save it and copy the mq4 file to the broker's MT4.
Restart Zorro and MT4.
Drag Zorro EA to a chart of MT4.
Press Trade and it worked.

TRY IT!