You have to provide it 2 arguments: the name of the executable to embed and the name of the VBscript to generate, like this:
file2vbscript cmd.exe cmd.vbs
This will generate a VBscript that will write cmd.exe to the current directory and execute it (create a new process). If you want to load a DLL in stead of executing an EXE, use the -l option:
file2vbscript -l mydll.dll mydll.vbs
And to use it in an Office application (Office VBA doesn’t take long subs), use the option -o:
file2vbscript -ol mydll.dll mydll.vbs
This will split the embedded file over several subs, to accommodate for the size limitation of Office VBscripts.