TNSnames Editor button of Database Login window is grayed out.
Misplaced or incorrectly named TNSnames.ora file.
Ensure TNSnames.ora file is located in the %ORACLE_HOME\NETWORK\ADMIN folder i.e. C:\oracle\product\10.2.0\client_1\NETWORK\ADMIN
Ensure TNSnames.ora file is not named incorrectly with any other characters or different extensions.
(See screen shots: Correct ORA Files.doc)
Ensure that the extension for the TNSNAMES is ".ora." Windows by defult do not show entension of know types and .ora maybe consider a know type. Enable view of extensions to check. For example, make sure the file is not TNSNAME.ora.txt.
If a TNS_ADMIN variable is set, check to be sure that path is valid.
Follow the steps for kb article 64853: Common Oracle Connection Issues...: