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Plugin Development
Talking about how to write plugins
If you want to know how to install a plugin, the following instructions might help: http://learn.elgg.org/en/1.11/admin/plugins.html#installation.
Regarding the postings you made on the "Online" plugin pages asking how to install them, please note that these "Online" plugins had been published already a while ago, i.e. they were developed and tested on older versions of Elgg and therefore might not work anymore correctly on the current most recent version of Elgg.
A simular plugin to display which users are currently online is to be found at https://community.elgg.org/plugins/1670251 and this plugin should work also on recent versions of Elgg (e.g. version 1.9.0 of the plugin on Elgg 1.9 or newer).
If you want to make a new plugin, read these guides: