If you are using the UGX patch then the riser function is broken, and you will need to fix it.
How to fix: http://ugx-mods.com/forum/index.php?topic=32.msg72#msg72
Half of that fix is pointless... In the beginning he is defining things that don't need to be defined for the script to work. The only thing that matters is removing the level.script checks for the Treyarch DLC names. I have edited the tutorial to remove this unnecessary step.
I also fixed the one line in the _spawners file and now it works fine - the wiki link has been updated with this new version. Some missing soundfiles have also been added.
And for crying out loud, report these things to a dev directly instead of just posting workarounds and "it's broken" in help topics. This doesn't fix the bigger problem and then later people run into the same issues again because they are still using the broken file. This could have been fixed as soon as it was discovered if the proper reporting had been done.
If there is anything ELSE that needs to be fixed, please tell me directly and I will fix it. It's been years since I've dealt with a stock installation and I am unaware of most of the idiotic problems left behind by Treyarch that break the basic functions of zombiemode.
@OP If you want to save some time, just download the fixed _zombiemode_spawner from this reply and replace the one youre using. The risers will be fixed after you recompile.
PS: You have about twice as many pathnodes as you actually need in that room you screenshotted.