I have a portal that allows creation of records in a new table. However, whenever I want to add new information in the portal of the other table, because I made the I.D. for that record unique, it won't let me edit that record because it think I am trying to create a new record...

How do I get Filemaker to realize that I want to edit the record with the i.d. I am using instead of trying to create a new record with that i.d.?

Not quite following your question. If you have allowed creation on then simply by clicking in the last portal row you are creating child records. To edit them, either go to a form view of the child record using a button that gtrr (the button must be in the portal row), or set the field behavior of the child fields in the portal to allow edit in browse mode.

As far as IDs go. There are three IDs you need. A ParentID in the Parent Table, a ChildID in the Child Table (both unique, auto-enter serial) and also in the child table, a _kF_ParentID. The relationship btw the Parent and Child table is Parent::Child by Parent::ParentID=Child:_kF_ParentID