How To Enter Data Into A Custom Drupal Entity?
I think everyone suffers the same bewilderment on first meeting ECK. Its workflow is bonkers.

You begin by creating an entity type and optionally renaming the default bundle (which otherwise will have the same name as your entity). I recommend giving the bundle a unique name to avoid confusion in this rabbit warren interface.

Then from the list of entities, click into that entity and then into the bundle. From here you can manage fields - and add your data from the 'entity list' tab. There, the link to add new instances of your entity type will be at the top of the interface - "+ add [name of your bundle]" and the list there will effectively serve as a rudimentary content list for that entity until you can whip something up in views. 

If you want to cut through the labyrinth and use a simple link to add your data, it'll be structured like this:

admin/structure/entity-type/[your entity name]/[your bundle name]/add