You can't add new rows in the designer. The "Add" link is only there for users to add rows when filling out the form, and generally-speaking each row must have the same fields/configuration.

Think of tables and collections as repeating patterns. In the designer you're setting up a base pattern/row which can then be duplicated any number of times on the form.

I'm not sure exactly what you mean when you say you want to set the formatting, but the variables in a table/collection need to be consistent so they can be collected/stored correctly.