First, you should create a folder to be used as your mounting point (for example):
sudo mkdir /mnt/MyShare
Then, mount the shared folder using the name you set in VirtualBox for your share; for example, if you named your share "VirtualShare":
sudo mount -t vboxsf VirtualShare /mnt/MyShare
Hope that helps, cheers!
for centos
yum install kernel sources
yum install kernel-devel
yum install gcc
ln -s /usr/src/kernels/2.6.18-92.1.18.el5-i686 /usr/src/linux
sh VBoxLinuxAdditions-x86.run
