De manual on-line do VirtualBox :
4.2.3.1. Installing the Solaris Guest Additions
The VirtualBox Guest Additions for Solaris are provided on the same ISO CD-ROM as the Additions for Windows and Linux described above. They also come with an installation program guiding you through the setup process.
Installation involves the following steps:
Mount the VBoxGuestAdditions.iso file as your Solaris guest's virtual CD-ROM drive, exactly the same way as described for a Windows guest in Section 4.2.1.1, “Installation”.
If in case the CD-ROM drive on the guest doesn't get mounted (observed on some versions of Solaris 10), execute as root:
svcadm restart volfs
Change to the directory where your CD-ROM drive is mounted and execute as root:
pkgadd -G -d ./VBoxSolarisAdditions.pkg
Choose "1" and confirm installation of the Guest Additions package. After the installation is complete, re-login to X server on your guest to activate the X11 Guest Additions.