Yes the ovftool is used at the command line in either a Command Prompt (Windows) or a Terminal (Unix/Linux) and André has already provided you the basic syntax so I fail to see why you can't do this for yourself. There is also documentation at the link I previously provided. Specifically "To run VMware OVF Tool from the command line" on page 19 of OVF Tool User Guide.
The FGT_VM-v400-build0482-FORTINET.out.ovf.zip file you attached contains the following files:
Fortigate-VM.ovf
Fortigate-VM.hw04.ovf
Fortigate-VM.hw07_vmxnet2.ovf
datadrive.vmdk
fortios.vmdk
There are 3 different .ovf files which create 3 different .vmx configuration files for 3 separate VM's that use the same virtual hard disks, although separate copies for each VM.
I extracted these files to a folder on the C: drive named FGT_VM so in a Command Prompt I changed to that directory and the used the FQP of the ovftool and supplied the command line with the following syntax:
ovftool <options> <source locator> <target locator>
For <options> I used --acceptAllEulas so I wouldn't have to page down through and accept the EULA by typing "yes" and chose the Fortigate-VM.ovf as my <source locator> and Fortigate-VM.vmx as my <target locator>.
Using the following commands as shown in the image below it created the following files and these are the files I would then place by themselves in their own folder for the first VM. (If you want to process the other .ovf files to create the other two VM's the process is the same.)
Fortigate-VM.vmx
Fortigate-VM-disk1.vmdk
Fortigate-VM-disk2.vmdk
(Click image to enlarge.)
You do not need to worry about the "No manifest..." warnings, in this particular case. Just gather up the created files to use for the VM! 
Message was edited by: WoodyZ - Added link for OVF Tool User Guide.