I was having the same issue with my NVIDIA GPU and a google search lead me here so I thought I might share my solution.
My problem was that I added the java and javaw applications that were in /Program Files (x86)/java/bin, but after checking in the task manager, I found out it was using the applications located in /Program Files/java/bin. After adding those my GPU is being used whenever I launch Tekkit or Voltz.
I hope this can be of help to anyone having the same issue with Tekkit and/or Voltz. And remember, if you have the program open, you can always use the task manager to find the location of the .exe by right clicking the task you are curious about.
