That’s not an unusual warning and is benign and can be ignored for now.
OH will see the file and load it automatically, no need to restart anything. You should see a line something along the lines “loaded model nameOfFile.rules” as soon as you save the file. There is a file system watcher.
Assuming you put the file in the right folder, which I have no reason to doubt, there does seem to be something off with Rules DSL. I’m not sure what it could be though. There is nothing to configure or install to make it work. Rules DSL should just be there.
If you log into the karaf console (from the machine running openHAB run openhab-cli console if it’s installed or use ssh -p 8101 openhab@localhost using "habopen as the password. Then run list | grep -i rule. Are all the bundles shown as “Active”?
If not or as a last resort you can try to Clear the Cache and see if that makes a difference.