I have the solution!! Duh Duh Duh DUNNNNN! (For Macs Only)
YAY!
Go into your library by clicking on finder and then clicking go at the top of your screen and then clicking 'Go to Folder...' and if you have done this correctly, you should have a text field on your screen. Type in ~/Library inside the text field and slam that OK button! You should now be in your library! Find the folder Application Support and go into that folder. Scroll down until you find the technic folder. Go into that folder then go into modpacks, tekkit, bin and then finally the natives folder!
See the file named liblwjgl.jnilib? You will want to rename that to liblwjgl64.dylib (IT HAS TO BE EXACTLY THAT!) and remember to change the .jnlib to .dylib otherwise it will not work. Now launch Tekkit Classic the way you normally would. If you have followed my instructions carefully, it should work!
Happy to help,
TrooperPlaysMC