There should be a section in the owners manual on how to properly care for the underwater housing and apply the silicon to the o-ring. The big concern is keeping the o-ring "clean" and the groove that the o-ring seats in clean. The silicon is to be applied "very lightly" to the o-ring. Dont glob it on and it doesn`t need to be applied anywhere else on the housing unless there are specific recommendations by the manufacturer of the housing. Everytime you open the housing, it is a good idea to make sure the o-ring is clean and lubricated before you close the housing again.
That is the basics and I hope it helps. Just make sure to take the time and read the owners manual if you have it.