I developed Comparison Table plugin for Joomla. It has also one click update functionality but getting it work was quite a lot of work and debugging core joomla source code.
Official documentation for updateservers is here http://docs.joomla.org/Deploying_an_Update_Server
To make it short you must add updateservers tag to yuour plugin/component/module XML file
<updateservers> <server type="extension" priority="1" name="Comparison Table updates">http://marguspala.com/joomla/comparisontable_update.xml</server> </updateservers>
Next you need to put together update XML. Required tags there are name, element, type, folder(for plugins), downloads, and version. Element must contain very same text as you have in install XML package files in main filename plugin/module/component value.
<files> <filename plugin="comparisontable">comparisontable.php</filename> </files>
If you still get “There are no updates available at the moment. Please check again later.” then here is a few hints for debugging: Read more about Joomla one click updates in extension manager does not work for plugins …