Go to the main menu. Under "Devices" select "Install Guest Additions".
This is best done after installing GNOME, which should result in the ISO being automatically mounted. CD into the ISO directory:
# cd /media
Look for the location of Guest Additions directory/mount.
# cd <location-of-guest-additions-directory>
# pkgadd -d ./VBoxSolarisAdditions.pkg
Follow the prompts leading to the installation of Guest Additions.
Once completed, log out and log back in.