About Oracle 11g, one of the first things that caught me by surprise with 11g, when I first started in the beta program, was that the default location for the alert log has moved. It is still placed under the traditional OFA structure, but not /u01/app/oracle/admin. There is a new directory called diag that resides in /u01/app/oracle as seen on one of my systems:

rac13 | PWICRM3 | /export/home/oracle > cd /u01/app/oracle/diag/rdbms/pwicrm/PWICRM3/trace/

rac13 | PWICRM3 | /u01/app/oracle/diag/rdbms/pwicrm/PWICRM3/trace > ll alert_PWICRM3.log
-rw-r----- 1 oracle dba 278929 Sep 15 08:39 alert_PWICRM3.log

In this case, my database is called PWICRM and the first instance is PWICRM3. To quickly locate alert logs associated with many different ORACLE_HOMEs simple execute the  adrci  command and then execute “show alert”

rac13 | PWICRM3 | /export/home/oracle > adrci

ADRCI: Release 11.1.0.7.0 - Production on Thu Sep 15 08:50:52 2011

Copyright (c) 1982, 2007, Oracle.  All rights reserved.

ADR base = "/u01/app/oracle"
adrci> show alert

Choose the alert log from the following homes to view:

1: diag/rdbms/pwiprv/PWICRM3
……………………………………
6: diag/asm/+asm/+ASM3
Q: to quit

Please select option:

Also, you can get more helpful information via "help show alert":
adrci> help show alert

  Usage: SHOW ALERT [-p ]  [-term]
                    [ [-tail [num] [-f]] | [-file ] ]
  Purpose: Show alert messages.

  Options:
    [-p ]: The predicate string must be double quoted.
    The fields in the predicate are the fields:
        ORIGINATING_TIMESTAMP         timestamp
        NORMALIZED_TIMESTAMP          timestamp
        ORGANIZATION_ID               text(65)
        COMPONENT_ID                  text(65)
        HOST_ID                       text(65)
        HOST_ADDRESS                  text(17)
        MESSAGE_TYPE                  number
        MESSAGE_LEVEL                 number
        MESSAGE_ID                    text(65)
        MESSAGE_GROUP                 text(65)
        CLIENT_ID                     text(65)
        MODULE_ID                     text(65)
        PROCESS_ID                    text(33)
        THREAD_ID                     text(65)
        USER_ID                       text(65)
        INSTANCE_ID                   text(65)
        DETAILED_LOCATION             text(161)
        UPSTREAM_COMP_ID              text(101)
        DOWNSTREAM_COMP_ID            text(101)
        EXECUTION_CONTEXT_ID          text(101)
        EXECUTION_CONTEXT_SEQUENCE    number
        ERROR_INSTANCE_ID             number
        ERROR_INSTANCE_SEQUENCE       number
        MESSAGE_TEXT                  text(2049)
        MESSAGE_ARGUMENTS             text(129)
        SUPPLEMENTAL_ATTRIBUTES       text(129)
        SUPPLEMENTAL_DETAILS          text(129)
        PROBLEM_KEY                   text(65)

    [-tail [num] [-f]]: Output last part of the alert messages and
    output latest messages as the alert log grows. If num is not specified,
    the last 10 messages are displayed. If "-f" is specified, new data
    will append at the end as new alert messages are generated.

    [-term]: Direct results to terminal. If this option is not specified,
    the results will be open in an editor.
    By default, it will open in emacs, but "set editor" can be used
    to set other editors.

    [-file ]: Allow users to specify an alert file which
    may not be in ADR. must be specified with full path.
    Note that this option cannot be used with the -tail option

  Examples:
    show alert
    show alert -p "message_text like '%incident%'"
    show alert -tail 20

We can also get info detail from following oracle link:
http://download.oracle.com/docs/cd/E11882_01/server.112/e10701/adrci.htm#SUTIL700