The .vmx is located in a package. You have to open the package to find it. If you know the folder the VM image is stored in, open that folder, find the VM, right click and select "Show Package Contents."

If you do not know where the VM is stored, look in {Your User Name} > Library > Application Support > VMware Fusion > Virtual Machines. If it is a Boot Camp partition, then Boot Camp > {partion name} > Boot Camp.

When you find the .vmx, right click and open in TextEdit. Add the following line: