To start with, you need to ensure that the .vmdk file has the appropriate format for ESXi. You can use the vmware-vdiskmanager command line tool to convert the .vmdk file to an ESXi compatible format. This will result in two files, the header/descriptor .vmdk file and the data ...-flat.vmdk file.
If you don't have vmware-vdiskmanager available (it's included in e.g. VMware Workstation) you can try the version that can be found in the Attachments of http://kb.vmware.com/kb/1023856
Once converted you can upload the files to a datastore folder from the vSphere Client using the Datastore Browser.
André