If you haven't renamed lha_68k as lha yet, do so now. Assuming you have only one partition on your hard drive, try going to the enter a DOS command from the leftmost menu on Workbench. Type "lha x scorch.lha sys:" in the command box.
If it still can't open it then maybe the archive is corrupt. To view the files inside the archive type "lha v scorch.lha".