If you are trying to connect to an Oracle Database, through Server Explorer in Visual Studio, you have to ensure that:
1. You have installed the Oracle Data Provider (downloadable from the Oracle Website)
2. Have created the tnsnames.ora file, holding information about the databases (http://www.orafaq.com/wiki/Tnsnames.ora). This file should be created most often in the Network\Admin folder under the Oracle Provider. In my case: C:\app\user\product\11.2.0\client_1\Network\Admin
After having these prerequisites, I still couldn't connect to the Oracle Database. The Visual Studio wizard told me: "ORA-12154: TNS:could not resolve the connect identifier specified". What resolved this issue for me was to add the Environment Variable with name "TNS_ADMIN" and value the Path to the directory containing the "tnsnames.ora" file.
Then just restart Visual Studio and you're free to go.
Hope this helps someone out there :)