Board Thread:Questions and Answers/@comment-37.140.0.134-20140805062326/@comment-1757231-20140805170915

Are you on Tekkit Classic?

If not, next time go to the Minecraft forums. If you're using the Vanilla launcher, in "profiles" you can add Java arguments, which is where you would add them.

If you're on Tekkit Classic, I believe that the Launcher doesn't support Java args. You could contact the Launcher to get more info.