How to import export.sql in sql developer?
Open the file(s) it has created, and run them with F5 in SQL Developer. They should also work in SQL*Plus.

They'll create the tables/objects (if you have the Export DDL option checked), and then they'll INSERT the rows (if you have the Export Data option checked.)

Note that there are more efficient ways for moving objects/data around, especially when dealing with very large amounts of data - namely, Data Pump.

This particular feature is very helpful when wanting to do ad hoc data and object exports - especially when you, the developer, don't have access to run Data Pump.