Unable to randomize MAC address

[Trvekvlt]

Hi people,
I've created a 2VMs. One is with Windows server 2012 and the other one is with WIndows 7. I made the network "bridged" for both and I need to change the MAC address. I know where to do it. It's in the setting in the network section and in the advanced part and all but I'm unable to randomize it. Like, I can't click on it (it has pale gray color instead of black so it does nothing when I try). I can only change the "promiscuous mode" and select/deselect "cable connected". How to make it possible?

[Perryg]

The guest must be completely shut down to change the MAC address. Not saved state.

[Trvekvlt]

Now I feel stupid...
Thank you!!!

[Perryg]

Lots of people fall for this. Don't beat yourself up too much.