When the RIP sends the job to the DI hopefully the special colour will be allocated as X (make sure the RIP seperates it to X).
When you do the makeready procedure at the end before the final green tick ok it will say un-used seperations X, just tick ok. When you want to then print the special colour X just turn off the units, black, cyan, magenta and yellow and allocate the special colour X to the unit you want to print on, and just ok that the un-used seperations will be black, cyan, magenta and yellow.
Freelance Heidelberg demonstrator
based in the UK.