Sounds like you are trying to add new portal records by entering data into the portal. If you can't, open Manage | database | relationships and enable "allow creation of records via this relationship". If this is enabled and it still does not work, please describe the relationship. Relationships based on X instead of =, for example, don't permit adding new portal records this way and you have to then use another method.
See, I knew it was something stupid I was forgetting to do. Thanks Phil!