I would like to inform you that I have solved the problem with the turkish unicode alphabet.
The problem was not from PL/SQL Developer, but from the settings of my system.
Here are the constraints necessary to make sure the data is populated correctly.
1- In registery HKEY_LOCAL_MACHINE --> SOFWARE --> ORACLE
there should be a String values "NLS_LANG" with this value:
TURKISH_TURKEY.WE8ISO8859P9
2- Same string value must exists at HKEY_LOCAL_MACHINE --> SOFWARE --> ORACLE --> HOME0
3- From Control Panel --> Regional and Language Options , The user must change the Language for the non-Unicode programs (System Locale) to "Turkish(Turkey)" and restart the computer. (it is in administrative tab in windows Vista)
The number 3 was my problem of not being able to see tables of turkish characters or type them correctly.
Best Regards,
Aram Azhari