I have custom object 'Subject_c' with 3 fields and I have created those objects by uploading a CSV file. Subject_c has a lookup relationship with Leads (Its general for the same user regardless of what lead he is viewing). I am able to insert a related list and I can see that the objects are created under Data Management/Storage Usage. But it shows blank under related list.
You're saying that the custom object has lookup to Lead but then you say Subjects are generic and somehow should be displayed on every Lead page? I don't think it'll work.
Stuff appears on related list only when field Subject_c.Lead_c will be populated with "this" Lead's Id. (please note I've made best guess at the field name). So you'd need to insert separate data for each Lead which can quickly blow your storage usage and will be a pain in the a$$ to maintain later. Is it only for displaying? Or do you plan to later capture some kind of survey results for each Lead?
If it's just for display I think you'll need to embed a Visualforce page in the Lead page layout to achieve that in a saner way. The subjects are specific to current viewing user? Or it's more like a general list, just 3 subjects for whole organisation?
P.S. "object" is like a table in normal database. I think you mixed a bit the difference between table and records / rows of data stored in it.