The only way I know of to create or edit an import spec is during an import.
From the File menu select Get External Data. Pick the text file you wish to import and click the import button. (It must be *.txt, *.csv, *.tab, or *.asc to define an import spec) The Import Text Wizard will open. Click the Advanced... button to open the Import Specification dialog. Click the Save As button to give the spec a name so you can recall it later.