emerge nvidia-kernel   I get this error

>> md5 src_uri ;-) NVIDIA-Linux-x86-1.0-6629-pkg1.run
 * Determining the location of the kernel source code
 * Unable to find kernel sources at /usr/src/linux
 * This package requires Linux sources.
 * Please make sure that /usr/src/linux points at your running kernel,
 * (or the kernel you wish to build against).
 * Alternatively, set the KERNEL_DIR environment variable to the kernel sources location

!!! ERROR: media-video/nvidia-kernel-1.0.6629-r5 failed.
!!! Function linux-info_pkg_setup, Line 534, Exitcode 1
!!! Unable to calculate Linux Kernel version
!!! If you need support, post the topmost build error, NOT this status message.