The answer about going under "Other Users" was close, but not nearly explicit enough, so I felt the need to add this answer, below.
In Oracle, it will only show you tables that belong to schemas (databases in MS SQL Server) that are owned by the account you are logged in with. If the account owns/has created nothing, you will see nothing, even if you have rights/permissions to everything in the database! (This is contrary to MS SQL Server Management Studio, where you can see anything you have rights on and the owner is always "dbo", barring some admin going in and changing it for some unforeseeable reason.)
The owner will be the only one who will see those tables under "Tables" in the tree. If you do not see them because you are not their owner, you will have to go under "Other Users" and expand each user until you find out who created/owns that schema, if you do not know it, already. It will not matter if your account has permissions to the tables or not, you still have to go under "Other Users" and find that user that owns it to see it, under "Tables"!
One thing that can help you: when you write queries, you actually specify in the nomenclature who that owner is, ex.
Select * from admin.mytable
indicates that "admin" is the user that owns it, so you go under "Other Users > Admin" and expand "Tables" and there it is.