Go to the data source of the form or report. If the data source is a table, go to the design view of the table and add the field. If the data source is a query, add the desired field to the query if it is in a table on which the query is based. If it is not add it to the table and then add it to the query. The field will then appear on the field list.
You cannot add a field to a form or report if it is not on the field list directly. You can create a textbox which uses the dlookup function to lookup a value from a table or query which is not linked to the report or table.