The drop down in the control source property will list all the fields in the
form's record source property.  If the record source property of the form is
a table, it will list all the fields in the table.  If it is a query, it will
do the same.
If the field is not in the list, that means field does not exist in the
record source.  If the record source is a table and and the field you want is
in another table, you will have to create a query that joins the two tables
and change the form's record source to the query.  If the record source is
already a query, then there are two possibilities,  either the field exists
in one of the tables in the query but is not included in the query.  In this
case, edit the query and add the field.  Or, it could be you are still
looking for a field that is not in the tables in the query.  Now you have to
add the other table.
The only issue here is be careful you don't end up creating an uneditible
recordset.