Oh sorry,

If you are trying to add it from the backend, you have to add it based what the primary field of that data type is.

For example, in my database for users the email is the primary field, so I would click add, type out an email and it will auto populate a list based on emails.

If I had it set up for the primary field to be the UID, you would have to know the specific UID of that user to enter into that field, when you start typing it, it should populate a list and you can select the one you would like.