Is this a FreeBSD install or a PCBSD install? I'm going to guess it's a FreeBSD one.
When you installed, it would have depended upon what you selected during installation--perhaps you didn't install the source tree. I forget the exact choices, but perhaps you just chose to install the binaries.)
So, a cvsup should fix that.
RELENG_6 will give you PRERELEASE. RELENG_6.0 will probably put you a bit behind, getting 6.0-psomething. See
for a good explanation.
Once you do the cvsup you should have your source tree.
For the actual steps for installation, you can look at
Avoid the -j flags, according to the latest UPDATING.
Doing it the way you have it, make buildkernel make isntallkernel will install the GENERIC kernel, not a custom kernel.
After installkernel, reboot, preferably in single user mode.
mount -u /
(This might be important right now, as a new group, auditor, was added and installworld will fail otherwise.)
Again, see the link I posted above for more detail