to catalog z/OS databases you are using the subsystem name.
this can have more than 8 characters, correct.
Still: you need DB2 Connect for any LUW DB2 to access host data.
in DB2 connect you will have a dcs entry mapping the long name to something short with 8 characters being used finally.
First: ensure you have DB2 connect.
Second: for host you need one additional catalog statement: catalog dcs ... to the usual catalog database, catalog tcpip ,
third: if you dont feel sure about the statements, may be using the GUI of the Configuration Assistant (db2ca in V8, db2cca in V7) helps.
I suppose you are not using a connect server instance at a different machine ?