Note: Having clearly labeled import and export features for Base has been proposed and you can vote for it by clicking one of the following links. Here's how to make things work until the features are implemented and put into the next build.
Vote for adding a wizard to import data into Base
Utility to export CSV from Base
Here's the related article on the cleverly disguised import function, for getting data from a spreadsheet, Access, or another database into an OpenOffice Base database.