You do this by clicking on Create a New Virtual Machine, and then select Virtualize.
From the list of operating systems, choose Other, then click Browse… and find your Kali Linux installer that you downloaded earlier.
Once selected, click Continue, and I bring the Memory slider to 2048MB. You can assign more, but I find 2GB to be more than enough.
As for storage, I find that 32GB is more than ample for Kali Linux.
Once you get to the Summary page, give the virtual machine a name, check the box next to Open VM Settings, and click Save.