Oracle 11g does not provide the iSQLPlus interface or the GUI SQL*Plus client (sqlplusw). Instead, it provides SQL Developer and APEX (Application Express).
To get a web interface for typing in SQL and getting the results, you can use the Oracle Enterprise Manager (OEM) web interface, accessible at: https://localhost:1158/em (assuming you've got Oracle installed on your computer).
To log into the OEM web interface, you need to have the SELECT ANY DICTIONARY privilege (or you can use the SYSTEM account :-P ). When you're logged in, click on the Data Movement 'tab' and click the SQL Worksheet link (under the related links section). You'll now have the iSQLPlus-like interface for executing SQL queries.
EDIT: If you're using Quest SQL Navigator, you will need version 5.5.4 or later to work with Oracle 11g; this version also supports Microsoft Windows Vista.