Open Parallels
Click File -> New
Click "Add Existing Vitrual Machine"
Click Continue
Next navigate to the vmx image and select it
Click Open
It will say it needs to convert it, click convert. Choose the folder where it will be stored and click ok.
Depending on how big the disk your trying to convert it, it might take a while.
It is recommended to accept all suggestions from Parallels at least until the virtual machine is up and running.