Do add a new plugin that you would like to sell using "Upgrades", you need to place the plugin in the "/wp-content/mu-plugins/upgrade-plugins/" folder.
I downloaded a plugin from Wordpress (http://wordpress.org/extend/plugins/tweet-this/) and placed it into this folder. But the plugin does not show up when creating a new Upgrades package.
So what other steps must be taken?
NOTE: the three sample plugins (ad_removal.php, privacy.php and quota.php) are all one file. The plugin I dropped in is a complete folder. Would that make a difference?