Use uname -r to check that the kernel version in that path matches the kernel version you are running.
Also note that if you changed anything important in your kernel and recompiled it you might need to recompile the external module as well. What "important" means depends on the module itself, and the pieces of the kernel it uses. Also, always double check that the /usr/src/linux symlink is pointing to the right kernel tree (the one that uname -r says you are running). -- Jesús Guerrero Botella
