Relic released the CoH modding tools back on the 26th of March. However, it looks like they’ve found some issues with them, so have temporarily taken them out of service.
You can read JD’s post here.
For easy reference, here’s a quote.
Hello to all modders and fans of Company of Heroes,On March 26th, updated versions of our development tools were made available to the community. Unfortunately these were released prematurely and we have taken them down.
We are working to make them more stable as well as generating documentation in order to give you the level of support you deserve. Our time frame for this is as soon as possible.
We apologize to our loyal fans that have waited a long time for these and promise to make it up by giving you the tools you deserve.
Thanks!
Soon they should be back!