Oracle Database Discussions

Dear oracle community,

just stepped today into oracle world.
Installed OracleXE on my remote UBUNTU server.
From the server, sqlplus works perfectly.
From my client (followed this tutorial http://www.danilovizzarro.it/?p=10 ), i can connect via the
sqlplus connection string: #sqlplus hr/PASS@HOSTipADDRESS/XE .

But when i try just sqlplus and enter username/password later on, i get
ORA-12162:TNS:net service name is incorrectly specified .

My config:
OS - MAC OS X LEOPARD 10.5.5
#echo $ORACLE_SID
XE
#echo $ORACLE_HOME
/opt/oracle/instantclient_10_2/
#echo $TNS_ADMIN
/opt/oracle/tns/
# cat /opt/oracle/tns/tnsnames.ora

XE =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = HOSTipADDRESS)(PORT = 1521))
)
(CONNECT_DATA =
(SID = XE)
)
)


Any ideas - is it that sqlplus doesn't locate my tnsnames.ora file, or, maybe, there's an error in it?
thanks for any ideas, recipies, suggestions :)

enjoy the day,
james