The correct way to present the disks to ASM

This assumes that you've already partitioned the presented disk(s) (and will be using /dev/sd[whatever]N), and that you're using asmlib. There will be a kernel module loaded if you are:

[root@oel61 disks]# lsmod | grep oracle
oracleasm              53865  1 
[root@oel61 disks]# 

As root, scan for candidate disks:

[root@oel61 ~]# /etc/init.d/oracleasm scandisks
Scanning the system for Oracle ASMLib disks:               [  OK  ]
[root@oel61 ~]#

Then, check to see if the disk has already been "discovered" by ASM:

oracle@oel61 ~]$ asmcmd -p
ASMCMD [+] > lsdsk
Path
/dev/oracleasm/disks/DISK1
/dev/oracleasm/disks/DISK2
/dev/oracleasm/disks/DISK3
ASMCMD [+] >

If not, we need to stamp the device:

[root@oel61 ~]# /etc/init.d/oracleasm createdisk NEWFRA /dev/sdc1
Marking disk "NEWFRA" as an ASM disk:                       [  OK  ]
[root@oel61 ~]#

Scan for candidate disks again, then list - the new device should be there:

[root@oel61 ~]# /etc/init.d/oracleasm scandisks
Scanning the system for Oracle ASMLib disks:               [  OK  ]
[root@oel61 ~]#

# /etc/init.d/oracleasm listdisks
DISK1
DISK2
DISK3
NEWFRA
#

Or use asmcmd:

oracle@oel61 ~]$ asmcmd -p
ASMCMD [+] > lsdsk
Path
/dev/oracleasm/disks/DISK1
/dev/oracleasm/disks/DISK2
/dev/oracleasm/disks/DISK3
/dev/oracleasm/disks/NEWFRA
ASMCMD [+] >

Now do a scandisks on the other node and check that everything is as it is on the first node (should be fine if you're using the same /dev device names).

Now the disk is ready to be added to the group.

List the groups:

[oracle@oel61 ~]$ export ORACLE_SID="+ASM"
[oracle@oel61 ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.2.0 Production on Thu Jan 31 15:35:27 2013

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Automatic Storage Management option

SQL> select name
  2  from V$ASM_DISKGROUP;

NAME
------------------------------
DATA
FRA

SQL> 

Add disk to the group:

SQL> ALTER DISKGROUP FRA ADD DISK '/dev/oracleasm/disks/NEWFRA';

Mat is right. ASMlib is only one way to created candidate disks for ASM. I would recommend you use udev as detailed here.