On a standalone database, it is quite easy to enable the Archive Log mode, but in a RAC environment, there are more things to do.
It is the same thing on a 10g or 11g RAC Database.
To do the job, you need to stop cluster mode, stop database, enable the archive log mode et restart all.
 
 

• Disable cluster mode on node one, by setting the parameter cluster_database

$ sqlplus / as sysdba
SQL> alter system set cluster_database=FALSE scope=spfile sid='DBRAC1';

• Stop instances of the database

$ srvctl stop database -d DBRAC

• Mount instance one and enable archive log

$ export ORACLE_SID = DBRAC1
$ sqlplus "/ as sysdba"
SQL> startup mount
SQL> alter database archivelog

• Re enable cluster mode on the instance

SQL> alter system set cluster_database=true scope=spfile sid='DBRAC1';

• Stop instance one

SQL> shutdown immediate

• Restart all instances of the database using srvctl command

$ srvctl start database -d DBRAC

• If needed, restart all services

$ srvctl start service -d DBRAC

• Check if archive log is enabled

SQL> archive log list
Database log mode              Archive Mode
Automatic archival             Enabled
Archive destination            USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence     65
Next log sequence to archive   66
Current log sequence           66