You generally get that sort of message when the executable is not in your "PATH" environment variable. An easy way to get around this is to go to the folder where gba_mus_ripper is located and hold down the shift key on the keyboard. Then right click and there should be an extra menu option called "Open Command Prompt Here". Alternatively you can use the "cd" command after opening a command prompt and navigating to the folder that gba_mus_ripper.exe is in. Assuming you launched a command prompt that starts you our at C:\Users\Jake, and the exe file is in C:\Users\Jake\Downloads\gba_mus_ripper, here's an example:
That should be enough you get you started but you will likely have to provide parameters to gba_mus_ripper.exe that specifies which files you want to rip from. I assume the program has a readme that fills in those details.