Actually, it’s quite simple by following the steps below:
Step 1: On your host OS, macbook pro in my case, go to VirtualBox->machine->settings->Shared Folders, add the folder, say folder1, you want to share with your guest OS.
Step 2: On ubuntu 9.10, create a new folder, with name, say folder2, then edit your /etc/fstab file, add the following line
folder1 ~/folder2 vboxsf rw,gid=1000,uid=1000,auto 0 0
Then reboot your guest OS, then you will see folder1 is automatically mounted.
Note, the names of two folders must be different.