How to use dmg2iso on win7 professional?

Hi, I'm trying to convert a .dmg file to.iso file and I know you have to use the command prompt to open dmg2iso. I have both the .exe and .dmg file in the same directory so I put "C:\Users\User\dmg2iso\dmg2iso.exe filename.dmg filename.iso". I press enter and I get "cant open input file" is there anything I did wrong or is the file corrupted?