It seems you don't have descriptor VMDK(*.vmdk) file along with *.flat.vmdkfile. Please note that VMware ESXi will not recognize *.flat.vmdkfile while you attaching virtual disk with VM. This file contains only actual data of VM. You need *.vmdkfile(descriptor) for the particular *.flat.vmdkfile(data file) to create a VM in your VMware environment. Please refer the following VMware KB article to create a descriptor file manually and you can then create VMusing it.