what is the URL for JetBrains IDE plug-in repository?
I had the same problem while using the Community edition and trying to install the lombok plugin. Setting the HTTP Proxy to "Auto-detect proxy settings" resolved it for me.
It's under browse repositories -> HTTP Proxy Settings -> HTTP Proxy -> Auto detect proxy settings.
Manage repositories] list should be empty, unless you have some private plugins repository.
Browse repositories plugins] button should show you all the plugins, if not then check your firewall or proxy settings, or click on the Reload button.
I don't know the specific URL, either, but Meo's answer is correct in saying that there's no need to add it manually. It sounds like you're confused by the difference between the list you see on the site and the list you see in IDEA CE.
The reason for the difference isn't a missing URL at all, but rather that many of the plugins listed on the site are not enabled for the Community Edition, so it doesn't display them. The PHP plugin at https://plugins.jetbrains.com/plugin/6610 is an example. While that page doesn't make it clear that it requires the Ultimate Edition, you can find out by scrolling down to the Updates list and clicking the (latest) version number. There you'll find a Supported Products list, where the Community Edition is listed as being incompatible because the plugin requires the "ultimate" module (a module that is only included in IDEA Ultimate Edition).