I downloaded aggregate 0.74 from here: http://docs.cacti.net/plugin:aggregate#aggregate
Extracted it and copied it to /var/www/html/cacti/plugins/aggregate on my Cacti server.
The README for this plugin explains:
I am using Cacti 0.8.7e and PIA 2.6.----[ Prerequisites
Before you install Aggregate, you need to have met the following pre-requisites:
...
3) Cacti 0.8.7x requires Plugin Architecture V2.x.
Apparently I don't need to edit my includes/global.php to enable this to work. It doesn't matter whether I do it or not, the plugin never appears in "Plugin Management".
The files are chowned to root:root like the other plugins.
Since the plugin isn't installed here, it isn't available to add in the realm permissions (just pre-emting some of the possible responses)
Any ideas from anyone?