You can read it in the User Manual section 17.8 (Sounds so much nicer than RTFM, don't you think).
Anyway, for short..
Menu Tools/ODBC Text Importer.
User/System DSN: Excel Files
Click 'Connect' button to select file.
In the 'Table/Query' pane select the listed tab (for CSV files this will be only one). In the 'Result preview' pane the data in the file will be shown.
Go to the 'Data to Oracle' tab and fill out owner/table_name. In the 'Fields' pane you can map the file columns to database columns and set datatype.
Push import button, et voil