virtualbox 下的linux挂载共享问题

Building the VirtualBox Guest Additions kernel modules???? [FAILED]
(Your system does not seem to be set up to build kernel modules.
Look at /var/log/vboxadd-install.log to find out what went wrong)

/var/log/vboxadd-install.log:
Makefile:23: *** Error: unable to find the sources of your current Linux kernel. Specify KERN_DIR=<directory> and run Make again..? Stop.?

??

#?yum?groupinstall?"Development?Tools"??#?yum?groupinstall?"Development?Libraries"??#?yum?install?unifdef?rpm-build ????export?KERN_DIR=/usr/src/kernels/2.6.18-164.15.1.el5-i686/ ????sh?./VBoxLinuxAdditions-x86.run?