If you want to use X Window System graphic shell in FreeBSD guest OS, you should configure it manually. Running automatic configuration command X -probeonly or X -configure may not work. X Window System can be configured using xorgconfig text utility or xorgcfg graphical utility. You need root privileges to run them.
To start manual configuration:
su -l root -c xorgconfig
or
su -l root -c xorgcfg
root password when you are asked for it.When performing configuration set the following:
Generic VESA compatible video card.4096K of video memory.You may set a single resolution for a specific color depth (for instance, if you are going to work with 16-bit color only, select any single resolution for it), and later select for your configuration only this specific color depth.