If you've forgotten your password you can't retrieve it. You can only reset it.
Log in to MyVerizon and click on the MyServices tab, then off to the left click on MyPhone. Once the page loads you'll see a My Phone Actions column that has "Reset Voicemail Password". Click on that and it'll help you reset it.
Or you can also do this by calling the Customer Service IVR at 800-922-0204. I believe it's option 3 or 4, but listen for the prompts to tell you which one to select for voicemail.