Error message - data type mismatch in criteria expression - in query

While you see a text value in what you call a 'drop down' field, this is not the actual value in the field.  The real value is a number which references the numeric primary key, usually an autonumber, of the referenced table.  To apply a criterion based on the text value you must join the table to the referenced table in the query and return the text column (field) from the referenced table.  You can then apply a text criterion to that column.

I imagine you used the 'lookup field' wizard when adding these columns to the table in design view.  This feature should be avoided, however.   For reasons why see:

http://www.mvps.org/access/lookupfields.htm

Seeing the value from the referenced table is unnecessary in a table's datasheet view as data should only be accessed via forms or reports, never in raw datasheet view.  Bear in mind that a form can be in datasheet view.  In a form you can build a combo box (or allow the control wizard to do so) which shows you the values from the referenced table in the same way as a 'lookup field' does, but without the undesirable consequences detailed at the above link.  In a report you can join the tables in a query and bind a control to the column in the referenced table.