I would recommend creating a TNSNAMES.ORA file. From your Oracle Client install directory, navigate to NETWORKADMIN. You may already have a file called TNSNAMES.ORA, if so edit it, else create it using your favorite text editor.

Next, simply add an entry like this:

MYDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 123.45.67.89)(PORT = 1521))
    (CONNECT_DATA = (SID = TEST)(SERVER = DEDICATED))
  )

You can change MYDB to whatever you like, this is the identifier that applications will will use to find the database using the info from TNSNAMES.

Finally, login with MYDB as your database in PL/SQL Developer. It should automatically find the connection string in the TNSNAMES.ORA.

If that does not work, hit Help->About then click the icon with an "i" in it in the upper-lefthand corner. The fourth tab is the "TNS Names" tab, check it to confirm that it is loading the proper TNSNAMES.ORA file. If it is not, you may have multiple Oracle installations on your computer, and you will need to find the one that is in use.

In the "database" section of the logon dialog box, enter //hostname.domain:port/database, in your case //123.45.67.89:1521/TEST - this assumes that you don t want to set up a tnsnames.ora file/entry for some reason.

Also make sure the firewall settings on your server are not blocking port 1521.

The problem is not the TNS file, in PLSQL Developer, if you don t have the oracle installation, you need to provide the location of the OCI.DLL file.

In PLSQL DEV app go to Tools-Preferences-Oracle/connections-OCI Library.

In my case I put the next address C:OracleInstantClient-win32-11.2.0.1.0oci.dll.

If have Weblogic app installed, I didnt tried but if you want try to put the next location

C:OracleMiddlewarewlserver_10.3serveradr.

In addition to Richard Cresswells and dpbradleys answer: If you neither want to create a TNS name nor the //123.45.67.89:1521/Test input works (some configurations wont), you can put

(DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = 123.45.67.89)(PORT = 1521)) (CONNECT_DATA = (SID = TEST)(SERVER = DEDICATED)))

(as one line) into the database section of the login dialog.

I am facing to this problem so many times till I have 32bit PL/SQL Developer and 64bit Oracle DB or Oracle Client.

The solution is:

1. install a 32bit client.
2. set PLSQL DEV-Tools-Preferencies-Oracle Home to new 32bit client Home
3. set PLSQL DEV-Tools-Preferencies-OCI to new 32 bit home /bin/oci.dll For example: c:appadminproduct11.2.0client_1BINoci.dll
4. Save and restart PLSQL DEV.

Edit or create a TNSNAMES.ORA file in c:appadminproduct11.2.0client_1NETWORKadmin folder like mentioned above.

Try with TNSPING in console like

C:>tnsping ORCL

If still have problem, set the TNS_ADMIN Enviroment properties value pointing to the folder where the TNSNAMES.ORA located, like: c:appadminproduct11.2.0client_1 etworkadmin

Username : username
Password : password
Database : //123.45.67.89:1521/TEST
Connect as : Normal 

this work for me and (version 13.0.6.1911 64 bit)