Editor functions can only be used if KLayout runs in edit mode. KLayout can be put into editing mode by simply supplying the "-e" option on the command line:
klayout -e [<input file>] [-l <layer properties file>]
Accordingly, with the command line option "-ne", non-editable mode can be enforced.
On Windows, there are start menu entries for editor and viewer mode. Klayout can be configured to use editing mode as default when started. To enable editing by default, check the "Edit mode" check box on the "Application" tab in the setup dialog ("File/Setup").
In editing mode, some optimizations are disabled. This results in somewhat longer loading times and a somewhat higher memory consumption. The actual increase strongly depends on the nature of the input file: for example, OASIS shape arrays are not kept as such in editing mode and resolved into individual shapes.