What's wrong with how you installed it?
If you want to go back and forth between kernels, just open Zen Updater, install the latest update of the Updater (should be Version 2.1) and then you can go to EXTRAS in the Zen Updater and select different options. Included in those is the stock kernel and Toast's #65 OC Kernel.
I'm not sure what the directions above are referring to, but that's what I did. I installed Zen2.6.2and then flashed the kernel.
EDIT: After re-reading, it seems the poster above is saying flashthe 2.6.2OC ROM first, then flash the update (2.6.2) as described from the XDA thread, sa shown below:
Update for ZenEXP-0.9.6.2: This is not a full rom it's an update to 0.9.6.2. This update will replace some libs with updated versions as well as set the default min clock speed to 245 and the max to 691 and the cpu governor to ondemand so that there is no need to use setcpu or overclock widget unless you want to use custom values. This update will also fix voice search. Flash this through recovery. No wipe required. If you are doing a fresh install then Flash ZenEXP-0.9.6.2 first and then flash this update directly after without rebooting.
Also, once you flash the Zen rom, you can do almost everything from the Zen Updater on your phone (included when you flash it). You can roll back to older or experimental version of Zen as well.