You should reach out to Informatica for help here.

I am using trial version, so i dont have support.

both the trial version and the Free DataLoader are supported through the Informatica Cloud community.     Here is a walkthrough of getting your oracle configured.  http://iodusers.ning.com/forum/topics/configuring-iod   most likely this is a permission issue (unable to browse tables) or if you can't test connection you might have a conflicting tnsnames.ora on the same computer.