Hi Neil,
Here is an example of "Down in X" for a table your size. "Down in Y" would be similar.
Just click on the parameter that you want to change and it will be highlighted. Enter the value you want , then hit ENTER (like Hood said).
Unlike other wizards, the FINISH cut is ADDED to the TOTAL depth so you have to leave it at ZERO or you will get 2 passes ... unless that is what you want.
Use you own settings and feed rate, this is just an example.
Regards,
Russ