Need help storing Core Data entities!! Eeek!

When you create an instance the of Step entity, it is already placed in the store, although depending on the level you are implementing (Are you using a NSPersistentDocument-based model?), you might need to assign new objects to the particular store. Considering you seem to have no problems with the Assignment object, I will assume you know this already.

However, it does not setup relationships for you. If you haven't done this already, you need a one-to-many relationship to the Step entity from the Assignment entity, and then an inverse relationship to the Assignment entity in your data model. Once the data model knows about the relationship, you can then in your code, add the new entity as a 'child' of the Assignment entity. Once this is done and your data saved to the store, the relationships are also stored for you, and reconstructed when you open it back up.

As for the second question, there is a method that might be worth looking at: [NSTableView editColumn:row:withEvent:select:]. The description says that it is normally invoked by the UI, and shouldn't normally be invoked by the developer, but it seems to be exactly what you want. You just need to know the Column/Row of the new element, which might be tricky, but I don't know the complexity of your application.

Edit: As an aside, the way it seems you are using Core Data isn't really going to benefit you with this application and is more complex than you need. Are your assignments being dumped out to file on their own? If so, you should probably be using an NSPersistentDocument model if you want to continue using Core Data and not overly complicate your code. If you are trying to create something where you see all your assignments in a single window, then I would consider using a single data store for ALL your data (have it mirror how the user interacts with it, instead of creating this disconnect), or if you can't do that, then I would abandon CoreData and use the basic Cocoa data-types. You can write dictionaries/arrays out to plist files, and the plist files can be written in a binary format to disk (IIRC).