First you got to have all four items in the party's inventory (lover's ring, shaman's staff, blood of a friend,withered hand). Then, go to the 4 tubes as you did, and hover over each one with your mouse pointer until you get the "use" option (circle-like mouse pointer). Click on each of them, the first time you'll click on a tube you'll get a riddle-description on which item takes what place.
Then start trying combinations.
It shouldn't be much of a problem, the lightning damage you take is rather low and you can easily heal between tries if you want.
If you're really stuck i could just say the combination, but there wouldn't be much to that, would there?
