The built in method of plugins being able to be updated via the wordpress admin page requires you release your plugin to the wordpress open plugins platform.
If you have a plugin you want to keep private you have to find another way.
Private plugin updating resources
https://wordpress.stackexchange.com/questions/13/updates-for-a-private-plugin
USEFUL?
We benefit hugely from resources on the web so we decided we should try and give back some of our knowledge and resources to the community by opening up many of our company’s internal notes and libraries through mini sites like this. We hope you find the site helpful.
Please feel free to comment if you can add help to this page or point out issues and solutions you have found, but please note that we do not provide support on this site. If you need help with a problem please use one of the many online forums.