I am using Development Oracle 10g and pl/sql 9.0.6. The problem is that everything was working fine but since when I also installed development Oracle 6i reports I can't see "tnsnames.ora" in my pl/sql developer database. I configured the same tnsnames.ora file in oracle 6i as I configured in Oracle 10g. I've Installed Oracle 6i on c:\ drive and oracle 10g on d:\ drive. Does it creat the problem in configuration of tns files? Database part in the Tools>Preferences is not showing in pl/sql developer.
Create an environment variable named TNS_ADMIN that points to the directory of the tnsnames.ora file you wish to use. Installing another Oracle product with its own home can often confuse clients using tnsnames.