Richard, my De Vere 4x5 Dichromat has two light boxes for inside the head.

At the front of the head there is the little louvred slot that swings out to replace the light globe and check the filters.

In the middle there is a large cover that hides the light mixing chamber, you should have two, one for 4x5 and one for 6x7 (ish) and under. My cover is held firmly in place by two fridge type magnets that are quite strong, a bit of pulling is required.

I personally leave the 4x5 one in all of the time as I find it slows printing times by about a 1/3 of a stop, other than that there is nothing else you need to do.

Happy printing.

Ps:- is yours a tabletop or a free standing with the drop table?