Hi Rica,
No problem. Hmm. It sounds like something might be incorrect with the service name in the ezconnect string...I would double-check to make sure everything there is correct. Another thing to double-check is that the colons and semi-colon are in the correct place in the ArcGIS service text box.
Also, is there any information in the oracle listener.log?
For your other question, you can use oracle10g client with the ezconnect string too, just use sde:oracle10g instead of oracle11g in the syntax.