Sounds like you have something mounted on the root partition. That's probably why the partition tool is not letting you proceed - it's needs access to the unmounted partition before it can write any changes (which, if you're installing BT4 to hard disk/vm, is kinda required).
If you follow the HowTo instructions step-by-step, you shouldn't run into this issue. Choose "Linux>Ubuntu" as the the guest OS when creating the initial VMWare machine, and make sure your new machine meets the prerequisites for BT4.


