Installed pkg for OS X.
Found out from another page that JDK needs to be installed.
Installed JDK by going to the link via java in terminal.
reported java version is 1.8.0_66-b17
Had to run the uninstall.command in /Library/Application\ Support/Jenkins.
Then ran install again and got localhost:8080 showing the Jenkins page.
Went to Manage Jenkins and then Manage Plugins. Plugins list is blank!
Went to advanced clicked refresh and got the error noted above.
Clicking (show details) does nothing.
Update site is set to:
Tried other mirrors with no difference in behavior.