Like many things, there are several methods that could work. I think the most straightforward would be;
1) right click on the datacenter or cluster and start the New Virtual Machine wizard
2) choose "custom" on the first page
3) work through the wizard to the "select a disk" page
4) at the right you will see 4 radio buttons and you need to select "use an existing virtual disk"
5) choose the .vmdk you have and proceed
***you need to have the .vmdk copied to a datastore prior to the wizard running
***there are other methods. this is one of them, and is pretty straightforward
If this helps, please mark this as a "correct answer"
JL