This is something the iWork designers never gave much thought to. Then when Lion came along it became even more difficult. Your custom templates are stored with other, obscure, User Account files.
You first need to access your User Library folder. Open a Finder window, hold down the Option key and click on the Go menu on the Finder menu line. When the menu drops down, continue to hold down the Option key and select "Library". This is your User Library, distinct from the System Library.
In the User Library, navigate to: Application Support, iWork, Pages, Templates, My Templates.
Good luck.
Jerry