They are in "/var/log/.." and are text files/archives.
Dmesg-command is a binary in "/bin/dmesg", on contrast. Do not confuse.
I remember that if you use Fedora, there is systemd and now there should be different way/location. You can ask Google or IRC of Fedora how to capture your kernel and Xorg logs.
Thats basically what step above did - clean them, do steps leading to the problem, and then submit the logs with crash info.