Nerd Alert Ultra-specific nerdery.

After completing the steps below for your platform, TNS aliases will be available in Oracle SQL Developer in the "New/Select Database Connection" dialog by selecting "TNS" for the "Connection Type" combo box. These steps are only necessary if you can't or don't want to install the Oracle Client for your platform.

For Linux machines:
-get tnsnames.ora from: https://mumbles.imt.uwm.edu/dbdoc/tnsnames-noworld.php
-copy the file to: /etc

For Windows XP machines:
-get tnsnames.ora from: https://mumbles.imt.uwm.edu/dbdoc/tnsnames-world.php
-copy to c:\tnsnames\ (or some other directory where it will never be moved)
-right-click "My Computer" and select "Properties"
-click "Advanced" tab
-click "Environment Variables"
-under "System variables" click "New"
-"Variable name:" TNS_ADMIN (must be caps)
-"Variable value:" c:\tnsnames\ (or whatever directory you used above)
-Click "OK"
-Click "OK"
-Click "OK"

Ref:
http://wiki.oracle.com/page/SQL+Developer+FAQ?t=anon

* How does SQL Developer find my TNSNames file?
o SQL Developer checks the following:
1) $HOME/tnsnames.ora
2) $TNS_ADMIN/tnsnames.ora
3) /etc/tnsnames.ora ( non-windows )
4) $ORACLE_HOME/network/admin/tnsnames.ora
5) Registry Key