Question on assembling first 2600 program

I am trying to assemble that calculator program for the Atari 2600, using DASM and Notepad as my text editor. Now, I am not quite sure what I am doing, so here's what I did (but it isn't working). I saved the calculator program. I saved it in the directory DASMbinDOS in the dasm22007 file. I then went as far into this directory as DOS in the command prompt window. I then typed in: dasm clock003.asm -f3 -oclock.bin . I have the calculator code saved as "clock003." The computer said, "Warning: Unable to open 'clock003.asm.' " And then, "Complete," on the next line. It made a .bin file called clock.bin, like the guide I'm using says, but this .bin file contains 0 kilobytes. What am I doing that is wrong? I also tried renaming the clock file as "clock003.asm" but that just gave the same result.

P.S. I originally posted this in the Programming forum, but then realized about the Atari 2600 Programming for Newbies forum, so here it is also.