Put the .el file into a directory in your LoadPath and add the following to your ~/.emacs file:
(autoload 'ddl-mode "ddl-mode" "A major mode for editing DDL files." t nil)
When you are visiting a DDL file, use ‘M-x ddl-mode’ to activate the mode.
For best performance you should byte compile .el files but it’s not required. A CompiledFile usually runs faster. There should be no warnings when compiling this file.
See TandemNskSupport for a method of automatically recognizing the language of Tandem files since they don’t have an extension and coders often can’t stick a -*-ddl-*- comment in line 1.