Just thought that this was worth sharing for anyone who uses BYOND in VirtualBox.

Due to a lot of the new features now requiring hardware mode to be enabled, anyone in VirtualBox can be put at a disadvantage as DirectX 8/9 isn't supported by default.

To correct this you can change the settings in your Guest OS to enable 3D Acceleration, boot the OS in safe mode and run GuestAdditions. Run the GuestAdditions CD and tick the enable DirectX support box during the install. When you reboot out of safe mode you will be able to take advantage of the new features in BYOND 510.

May seem simple enough, but after initially coming across this issue and with some Googling it looked like it wasn't going to work out. However I did a bit more digging and finally got it to work, so I hope this helps anyone who's experienced similar issues.
Okay. You're literally the only one that didn't know about this.