how about trying to read the nice handy walk through..
found here
http://www.bellaonline.com/gaming/w/spc/strategy_lib_spcwalk.html which says in the third paragraph for this level
Go up on the box by the open vent - checkpoint. Jump over onto hanging box. Jump up to cable and across. Hear radio chatter about '7th key'. Jump over to floor. Go to the poking-up pipe and rappel on wall. You see two guys over a dead body. Note you can't get in this way. Go over to skylight and jump down into the room.
Walk Throughs for most games are available on line and can easily be found by seaching for them using google
i.e. the search string
"splinter cell" walkthrough
gives the above link
enough of that, I am a Cat in a Hat
