When you create a new table, FileMaker automatically adds a "box" to Manage | Database | Relationships with exactly the same name. This "box" is called a table occurrence. It also creates a new layout with exactly the same name.
If you open Layout Setup... for a given layout, you'll find that one of these table occurrences has been selected in "Show Records From". The :: starting each field name is how FileMaker indicates that these fields do not come from the selected table occurrence. What information is shown in these fields and whether or not you can enter data into them will be controlled by how you have related the two table occurrences. You can't enter data in the fields from the related table occurrence because there is no new related record in your new table into which you can enter this data.
It may be that all you need to do is to double click the relationship line linking the two table occurrences and enable "allow creation of records via this relationship" for the table occurrence of the newly added table, but this assumes that the relationship you defined supports this.