Missing build

[McLinux]

Toshiba A110 - 224 laptop. It has a Realtek 8101E ethernet device.
Gentoo 2.6.17-gentoo-r7 kernel.
Now I am trying to install the driver.
I am not new to Linux but absolutely new to Gentoo.
My former distribution was Fedora Core 1 on my old laptop.
I downloaded the driver. r1000_v1.04
The README told to execute the following command:

#make clean modules

Now he complains:

/lib/modules/2.6.17-gentoo-r7/build: no such file or directory . Stop.

Then I downloaded the source of the kernel with the command:

emerge gentoo-sources

This was succesfull

Then I found an ebuild named: gentoo-sources-2.6.17-r7.ebuild.

#ebuild gentoo-sources-2.6.17-r7.ebuild install
installed the source in
/var/tmp/portage/gentoo-sources-2.6.17-r7/image/

But still no build directory.

How do I become one?

[psy_ill]

Ok, I recommend you to read the Gentoo Handbook and do not skip one part of it.
Read the part about introduction to Portage extra carefully.

More recommended literature:
Kernel Upgrade Guide
Kernel Config Guide

Be sure to check for support for the Realtek device in the kernel source (use "make menuconfig") before you try to use an extern driver.
You are of course welcome to ask if you can't figure something out, but please, RTFM!