On paper it looks completly right what youre doing but the error msg is something like "file not found" so please check again if you specified the file path corretly. My cmd ordering is a little bit different because i create the "empty" VM first so thats foldername already exisits when starting. I always start in the source VM folder.
cd /vmfs/volumes/datastorename/source_vm/vmkfstool -i source_vm.vmdk ../new_vm/new_vm.vmdk -d thin
When ever possible use the shell feature of tab-complete.
Regards
Joerg