Isn't it on one of the bottom Tabs when you're at the periscope station? I've modded my game so much I don't recall if I edited it in or if it's a stock feature.
Look at the Tabs though, it should have a Tooltip too if you hover over it.
Look at Hellguards post - walkthrough:
http://www.subsim.com/radioroom/showthread.php?t=231474
There are tutorials & video tutorials in one of the sticky's
http://www.subsim.com/radioroom/down...o=file&id=1647
Be sure to open your outer torpedo 'doors' before you fire, those few seconds delay will make you miss if you don't.
Happy Hunting!