For me the launcher is downloading and launching vanilla, regardless of auto setting or versions etc. I don't know where to find the 152ngt zip though, any help please?
First, find the FTB folder - you'll see the mod packs you've already downloaded (say Ultimate, TechWorld, etc....) - You should see 152ngt (in your case).
Unzip the 152ngt.zip folder somewhere (it doesn't matter where) - go into that folder, and you should now see a minecraft and instMods folder.
Now, go back to your FTB folder - and copy the two folders into the 152ngt folder there. You'll overwrite the minecraft folder (go ahead).
if it's not in the 152ngt folder (in your FTB directory), create a blank file named version - and add 0.5 as the first line. If it is there, edit the file (with notepad, wordpad - whatever) and add 0.5 as the first line, and save it).
Launch FTB, make sure 0.5 is set (not recommended) and it should now launch properly.
EDIT: tweaked a few things to (hopefully) make it a bit clearer.