Can you provide the exact error you are getting. If the error is "-bash: dcm2nii: command not found" then you want to make sure that you put the executable is in your path. Alternatively, you provide the path to the executable in the command line, e.g. 
  /Users/rorden/exe/dcm2nii /Users/rorden/DICOM
In general if you run my software without any parameters it will provide examples of how to use the software.

I would suggest you heed the warning generated by dcm2nii and dcm2niigui "upgrade to dcm2niix suggested". dcm2nii is no loger in active development. Whie I hope it is mature and stable, the manufacturers are continually developing their interpretation of the DICOM standard. Unless you are converting data that is >17 years old, I think dcm2niix should be your tool of choice. You can get dcm2niix by downloading MRIcroGL. MRIcroGL includes a graphical interface (Import/ConvertDICOMtoNIfTI). A nice feature of that interface is that it displays an example command line with example output filenames, allowing you to design your command line with the graphical interface.