I extorted data as csv file form somewhere and then i m trying to import this csv file into filemaker. But i want to import only those records which are not available means some specific records from csv to filemaker database.How to perform this?

If this is a one-time conversion, it would be easiest to import ALL records, then find the "not available" ones and delete the rest. You can do this in a temp table (or file) if you prefer - esp. if you already have some other data in the target table.

Actually this scrip will be in every two hrs.so flow of this script is like below.

sendevent(to export the data as csv file from abc site)->import this file in database (but want to import some specific records from that csv file )

Well, if you cannot control the output on the exporting side (always the better alternative), then validate the target field to accept only the "not available" value/s. Set the validation to 'Validate always'. Records that do not pass the validation will be skipped during the import.