Your entries from TNSNames.ora should appear here and you would select the one that you need to use.
Since the drop down does not seem to be working, it would indicate that VS2010 cannot find your Oracle home.
An easy solution to this is to create an environmental variable called
TNS_ADMIN
and place the path to the TNSNames.ora file you wish to you
as such
C:\app\MyDir\product\11.2.0\client_1\Network\Admin\
Close VS2010. Make the environment variable changes, then reopen, the drop down should then show you the available ora connections you may want to use.
(you can look @ this thread for a bit more info: http://forums.oracle.com/forums/thread.jspa?threadID=2213359&tstart=0)
hth