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When you create any type of Documentum project in composer for the first time in an empty workspace - the core project is unpacked from the install and added as a reference. The project navigator doesn't list DocumentumCoreProject in the Documentum perspecitive but it is in the workspace file system. Switch to the Java perspective and you will see it.
If I create a new workspace to do something like create a TaskSpace DAR, I always start by creating a dummy empty project so that the core will be added to the workspace. You can copy or import it as well. I then import the TaskSpace reference project.
Always turn off "Build Automatically" on the project menu and only do Clean builds on the project you are working - do not attempt to build either the Core or the Reference project.
You should not build the TCMReferenceProject. It is only there so you can import it into the Workspace and designate it as a Reference Project for your projects.
If you need to install it, you should download the TCMReferenceProject.DAR directly and install it using DAR Installer.