We need a little bit more info than that. If you did not follow this
guide initially try reinstalling the launcher by following the
guide exactly. After you follow that guide. Please post your console log in debug mode to pastebin.com and link to it here. This
guide should help you get the log.
From what you describe however, I wonder if you are using a Mac? If you are you should
downgrade to Java 6 or upgrade your
lwjgl to 2.9 Your files will be in the modpack folder, so you need to do this for each of the packs you wish to use.