BACKTRACK 5-KDE GUEST: guest additions issue

[fabfab]

Host: Windows 7 Ult x64 - 16 GB RAM - Nvidia GT 630 - AMD Athlon 2 X4 640
VM: Virtual Box 4.2.12
Installed guests: Ubuntu 12.10 + Backbox 2.05 + BT 5-KDE
Description of the issue:
I had no problem to install the Guest Additions with Ubuntu and Backbox: the VM run fine.
With BT5 I'm unable to install the Guest Additions: I follow the procedure:
1) sudo apt-get install dkms : done
2) exit from the terminal
3) available device: VBOXADDITIONS4.2.12
4) open with file manager
5) click on VBoxLinuxAdditions.run
6) in the terminal windows appaer the notification "building the main Guest Additions module: fail"
ERROR: your kernel headers for kernel 2.6.38 cannot be found at /lib/modules/2.6.38/build or /lib/modules/2.6.38/source.
You can use the kernelsourcedir option to tell DKMS where it's located, or you could install the linux-headers-2.6.38 package.
Failed to install using DKMS, attempting to install without.
I don't have much experience with Linux systems; for this reason I want to study them on a VM.
Please can someone give me a hint to solve the problem?
Thanks.

[noteirak]

Have you already read HOWTO: Install Linux Guest Additions?