It is nigh unto impossible to debug that which you cannot see, and you have provided very little. Why not use copy and paste to show exactly what you did and the exact result - full command line, and full response? And in your follow-on comments you mention a couple of other error messages, but again with no context.
Since you mention sqlplus and "run connect @", I will assume your command looked something like this:
C:> sqlplus scott/tiger@orcl
With this usage, the '@' is a delimiter indicating that what follows ('orcl' in this example) is the net service name. When sqlplus parses out the command line, it will take this value ('orcl') and look for a match in the local file 'tnsnames.ora'. By default it will be located in %ORACLE_HOME/network/admin. A typical entry might look something like this:
ORCL =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = somehostname)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = orcl)
)
)
Where the first line is the net service name that sqlplus is looking for. If the requested net service name is not found, you get 'ORA-12154: TNS:could not resolve the connect identifier specified '. This is a very definitive error that means exactly one of two things: 1) the file 'tnsnames.ora' could not be located, or 2) the file was located but there is no entry for the requested net service name.
So, you need to prove that there is a tnsnames.ora file in the expected location, and that it has an entry for the requested net service name.