When you first register with Netflix everybody gets 1 month free. If you have signed up to an eligible package then VM should email you a promo code that you enter in your account on the Netflix site to get the free 6 months. Netflix have a help page about entering promo codes here:
I believe it can take up to 5 working days from when your order enters the system for the email with the code to be sent, but it might be worth checking the spam folder of the email account you have registered with them to see if it got marked as spam.
If you haven't got the email then you can try calling VM (150 from a VM phone or 0345 454 1111) and they might be able to look it up on your account and resend it or read it out to you over the phone. Alternatively you can wait for the VM forum staff to get to this thread in a few days and they can look into this for you.
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out so my answers may be wrong If my answer solves your problem please mark it as helpful as it may help others My setup: Vivid 200 Optical fibre with Hub 3 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.