Not sure if anyone else has tried this, but these steps might work if you haven't installed VMWare Tools yet. I'm using VMWare Fusion 5.0.2 (I was on 5.00 and Unity was also greyed out and I thought upgrading would work but it didnt). You have to install VMWare Tools and for the longest time I thought it was trying to Install since it says Installing under the Virtual Machine in the Menu Bar. I actually had to go through more steps for it to really install.
On the Menu Bar of VMWare Fusion (once logged into the Windows Virtual Machine ), click "Install VMWare Tools", then on Windows (I'm using Windows 8) open up Computer folder (or My Computer I'm guessing in some of the older platforms) and locate the VMWare Tools drive (listed under drives or devices). Double Click and go through Install process. I chose Complete Install, not sure if you will run into problems with Typical but this Unity thing was driving me crazy so I figured I better just install everything. When install is complete, the virtual machine will restart, then you sign in again (I'm signed in with my MSN account instead of Local on Windows 8) it then popped up a message saying I needed to sign out and in back in to get some of the features working, so I just signed out and logged back in with my user name.
Checked the Menu Bar and Voila!! Under View it finally shows Unity!! I'm actually using it right now, so it definitely works. Don't really know what to say for those that do have VMWare Tools installed 
. Crossing my fingers and hoping Unity doesn't disappear, but so far everything is working well.
Hope this helps!