Using Oly C750uz with Epoque 150DS

Help appreciated!

I have an Olympus C750uz with PT018 housing and a borrowed ES150DS strobe with arm and fibre optic connector.

Any tips on how to set this lot up. My previous experience is limited but I used Aperture priority with fill in flash from the camera and some results (shallow water) were o.k. I'm thinking either M or A but any rules of thumb for the flash greatly appreciated.

Also another daft newbie question with the strobe on the arm I assume it needs to be angled diagonaly across the cameras axis but at how acute an angle? Will the cameras built in flash cast a shadow because of the strobe slave sensor and why does the focus light seem to trigger the strobe? I thought it was IR and the slave shouldn't trigger.

Many thanks