Oracle VM Virtual Box Guest Additions installation failure in Fedora 17

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I was trying to install Virtual Box Guest Additions in Fedora 17. This is the outpout I've got:

Removing existing VirtualBox DKMS kernel modules           [  OK  ]
Removing existing VirtualBox non-DKMS kernel modules       [  OK  ]
Building the VirtualBox Guest Additions kernel modules
The headers for the current running kernel were not found. If the following
module compilation fails then this could be the reason.
The missing package can be probably installed with
yum install kernel-devel-3.6.1-1.fc17.i686

**Building the main Guest Additions module                   [FAILED]
(Look at /var/log/vboxadd-install.log to find out what went wrong)**
Doing non-kernel setup of the Guest Additions              [  OK  ]
[root@linuxserver MaquinaBochs]#

The bold line is the one that refers to the problem. I looked to the log file vboxadd-install.log and this is what I've got:

/tmp/vbox.0/Makefile.include.header:106: *** Error: KERN_DIR does not point to a directory.  Stop.
Creating user for the Guest Additions.
Creating udev rule for the Guest Additions kernel module.

It says that KERN_DIR doesn't point to a directory, but as root user in terminal I wrote this:

KERN_DIR=/usr/src/kernels/3.6.2-4.fc17.i686
export KERN_DIR

With those lines it is supposed to point to the kernel folder but it doesn't do it. I'm running Fedora 17 LXDE with the kernel version 3.6.2-4.fc17.i686 in a VirtualBox Machine. The version of Virtual Box is: 4.1.18r78361. I downloaded the kernel headers needed for the Guest Additions installation.

So if some of you can help me, I really appreciate it.

Thanks in advance.