Wait, I just peeked at the source code. Looks like it needs a second ldr filename for the output.
Run this for help on command line usage.
"C:\Program Files (x86)\LDraw\LSynth\lsynthcp.exe" -h
Run this for help on command line usage.
"C:\Program Files (x86)\LDraw\LSynth\lsynthcp.exe" -h