I split the two posts up to make them easier to read and what not.
I had this problem myself, and it's trickier than the last one. I tried a lot of things: re-installing it, running as admin, installing ZC then installing MC over it, renaming minecraft.jar to zombiecraft.jar, and a few other things. None of them worked, and the solution was simpler than most of what I tried. It all stemmed from my not having the most recent version of minecraft.exe from http://www.minecraft.net
To fix (or at least what worked for me):
1) Make sure you delete all of the files from your "C:\Users\*your username*\AppData\Roaming\.minecraft" folder (or whatever your install folder may be).
2) Install a fresh copy of ZombieCraft (As of this posting, most recent version is 1.4.1).
3) Run ZombieCraft and update it. If you're reading this, the updating process will get stalled at 69% (Error (4): null).
4) Close the ZombieCraft window. If you try to run it again and say to continue without updating, it will stall near the end, at "Done Loading".
5) **Important** Download the most recent version of minecraft.exe from http://www.minecraft.net.
Link: http://www.minecraft.net/download.jspYES, THIS MATTERS
6) Run the new copy of minecraft.exe you just downloaded. Log in, and when it asks if you want to update, click yes. Allow it to fully update.
7) Close out of your MineCraft window. At this point, if you try to run ZombieCraft, you won't be asked about updating, but it will still hang up at "Done Loading"
8) FINALLY, install ZombieCraft again (this will be the second time). Install it the same as you did in step #2.
9) After that, when you run ZombieCraft, it won't ask you about updating and it won't stall at "Done Loading". It should work fine!
I don't know why the older versions of minecraft.exe didn't work for me, why the updater throws an error at 69%, or why the loader hangs at "Done Loading" after that, and I don't really feel like looking into it.