ISA Server, Publish FTP on DMZ

I am trying to publish a FTP server that is on the DMZ. On the ISA machine, when I publishing the internal (FTP) address as 192.168.x.x and the External as 216.210.x.x it says that the address is not internal. To get this to work I have to add 192.168.x.x to the ISA’s LAT. But I have been reading that you should not add DMZ address to the LAT. What do I do.