The main MonoDevelop add-in repository located at go-mono.com contains the core add-ins and several other add-ins which are being maintained by the MonoDevelop team.
The contents of this repository can be seen using the MonoDevelop Add-in Manager.
The manager can be started by clicking on the Add-in Manager option in the Tools menu. To see which add-ins are available for install, click on the "Install Add-ins" button. Notice that the manager will only display add-ins that you don't have already installed.
The install dialog has a combo box which allows you to select the repository where you want to install from. You should select either "All registered repositories" or "go-mono.com".
The go-mono.com repository should be registered by default. In case you can't see it, click on the "Repositories" button, then to "Add", and enter the following url: http://go-mono.com/md.