PL/SQL Developer can only connect to an Oracle database using what's known as a TNS Alias.
This is defined a file called "tnsnames.ora" that contains some text something a bit like this (shamelessly stolen from @Balazs's answer):
EDBDEV =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = edbdev.mycompany.com)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = EDBDEV)
)
)
This allows administrators to define database names and "hide" the IP address/service name details from end users. They say "connect to EDBDEV" and that's all you need to know.
The location of this file is defined by the variable "TNS_ADMIN". On a Linux system you'd find this by typing:
echo $TNS_ADMIN
On a Windows box you can find it by opening the command prompt and typing:
echo %TNS_ADMIN%
In SQL Developer, you can change the "Connection type" box to "TNS", and it will present you with a list of the TNS Aliases that it knows about - e.g. those that it found in the file:
%TNS_ADMIN%\tnsnames.ora
If for some reason you need to override this location, you can set the location of the TNSNames.ora file manually in the preferences:
Tools / Preferences / Database / Advanced
lnsrctl
(or similar - can't remember exact name - been a while since I've been on Oracle!)