There are a couple of ways of doing this using filters. One way is to set a filter to filter if field1 (or whatever your field name is) .in.("C","N") which will select anything in field1 with a value of 'C' or 'N'. Or you can have your filter read field1<>"D" or field1.not."D". Either one will select everything in field1 with a value not equal to 'D".
Hope this helps.