I had this issue a few times:
you get SONAR installed and the web client working fine (e.g. you can go to http://localhost:9000 and see the dashboard).
you have a project to analyze with a Maven POM, to which you add the sonar target as described here.
you start the Maven run and it returns in short order saying:
Can not execute SonarQube analysis: Unable to execute Sonar: Fail to download [http://localhost:9000/api/server].
Turns out for some reason this is a problem with the default Ruby install on OSX. The workaround is to use JRuby instead of Ruby, best done with RVM, e.g. rvm use jruby. Someone mentioned this online, and I cannot find the post now, but thanks.
I use Sonar with Homebrew, by the way, which has its log files at