Possibly you do not own any tables but need to enable any synonyms permitted to you.
Otherwise you may have an old version of SQL Developer that suffers from bugs in the JDBC driver it uses under certain circumstances. Are you using a recent version like 4.0.3 or 4.1? If so, then the problem is most likely due to not owning the tables rather than any JDBC issue.
I am using Oracle SQL Developer and am not able to see the list of tables, views, procedures, etc. on the navigation bar. When I click on the plus sign to expand one of these areas, nothing happens. What can I do to fix this?
A user will only see their own objects in the main nav tree.
You need to open the 'other users' tree and select a user to see objects owned by other users.