When you create a policy with a custom ADMX template which uses a registry key outside of the standard 4 recommended locations (below) and open up your group policy editor using gpedit.msc, the policy will not be listed and you will get no error. The workaround is to use the group policy editor which comes with the group policy management console (i.e. right click on a policy in the GPMC.msc and choose edit). This one wasted a bunch of my time wasted, so hopefully you don’t have to go through the same.
Recommended Group policy locations:
One other note - this bug appears to have already been fixed in Win7 & Server 2008 R2.
Any reason why a Group Policy Preference couldn't be used instead? Custom ADMX's should usually be unnecessary (and are kind of the devil).
Group policy preferences couldn't be used because they are just that - preferences, not permanent enforced settings. Custom ADMX files are very handy and what is recommended to use if you want to roll out a number of different registry keys to all of your clients. They really are fairly easy to deal once you start working with them and have your group policy infrastructure setup streamlined.