Is anyone aware of how to get the "+" sign next to a list field from within an opportunity record. I have a custom list field on the opportunity where I want to allow users to add values to the associated list directly from within the record. I know I have seen the "+" available before.

The user's role will need to have at least "Create" level permissions for the "Custom List" permission. This permission is under the "Setup" permissions category when you edit the role.

does the user have permissions to add records of that type?