To mount a shared folder in a OpenSolaris guest in VirtualBox, you have to install the Guest Additions and then mount the file system type vboxfs. The Usage is:
mount -F vboxfs <share> <mountpoint>
If you want to share the filesystem /share from the host to the guest, then first create the share. Go to “Shared Folders” in the Virtual Machine's Settings and click the little folder with the green plus on the right. 
