Vá para o terminal TrueOS
sudo su
cd /var/db/portsnap
portsnap fetch
cd /usr/ports
portsnap extract
portsnap fetch update
git clone https://github.com/freebsd/freebsd.git /usr/src //This will take about 30 minutes - 1 hour depending upon your internet speed
cd /usr/src; make clean //This will take about 1 hour
cd /usr/ports/emulators/virtualbox-ose-additions && make install clean //This will take about 5-6 hours
reboot
Após a reinicialização, ajuste a resolução da tela do TrueOS para o recomendado.