Hello mrbean,
When you save the custom error code file, it defaults to the user.lib\errors folder, as this is where the LabVIEW development environment will look when loading custom errors. This is mostly documented here:
There is one paragraph of particular interest:
When building an executable (EXE) or shared library (DLL) with the Application Builder, make sure to add the custom error file as a support file (under the Source Files tab). For an executable, the custom error file will be saved in the <executable destination directory>\user.lib\errors directory. For a shared library, the custom error file will be saved in the National Instruments\Shared\LabVIEW Run-Time\<version>\Errors directory. When distributing a shared library, make sure to install the custom error file in this directory.
This is true for LabVIEW 7.x and earlier. In LabVIEW 8.0, the build specification in the project has a check box "Copy error code files" that will accomplish the same thing for you when you are ready to distribute.
Hope this helps!