Vmware Converter is designed to allow you to migrate a running machine (this can be both virtual or physical), a VM from one vSphere enfironment to another or as you are trying to do from another VMware product, Hyper-V or assorted backup images directory to a ESXi host or vCenter enfironmenr,- did you review Section 4 of http://www.vmware.com/pdf/convsa_50_guide.pdf
The vmdk formats are different between ESXi and Workstation so hence the need to use a tool like Converter.