Minecraft – How to get the new minecraft launcher GUI

There doesn't seem to be any official way. Then again, modding the game by splaying open and modifying the internals of the .jar file was never really officially supported, either. The new launcher is, ultimately, part of a rather large series of changes to Minecraft, including making mods actually something the game supports via the new Resource Pack stuff, but until that time, modding is a bit more awkward.

However, there's still a way to do it. Go into the versions folder inside .minecraft, and copy the version you want to mod. Rename the copied folder, and both the files inside, to some new name, say "modded 1.blah". Lastly, open the .json file in a text editor, and change the line that will read something like "id": "1.6.1", so that the ID matches the new name. The new launcher should now show a version "modded 1.blah" in the list, but since that version doesn't exist on the Minecraft servers, it won't get overwritten. Splay open and tinker with the .jar file in there the same way you always would with the old launcher.

Note:
If you are using the new launcher with an older version of Minecraft, then the mods you are using may attempt to access files in the bin folder directly. If this is the case, you can simply create a folder named 'bin' in the .minecraft folder where they are looking for it, and add the files the mod needs in there. The mod should find the files, and carry on as it used to, not knowing the difference.

I was finally able to determine with the help of some of my forum friends that Minecraft's source code has been updated so as to not allow it from being played in an applet to prevent some inconspicuous bug whereby people who hadn't payed for the game were able to play it in an applet. This fortunately reinvigorated my efforts to determine what was wrong with playing the regular launcher. I was eventually told by the IRC chat for Minecraft that it was my ESET NOD32 antivirus which was causing the problem, and that I was forced to uninstall it in order to play Minecraft and install a different antivirus. I have elaborated on this in the comment I made below, but my problem is unfortunately not yet resolved. Here is where I posted the bug on the official Mojira bug posting site. https://mojang.atlassian.net/browse/MCL-887 However as the original problem has been solved I'll simply leave this as the answer.