When you try to install Cisco VPN client on a mac OSX 10.6.8 on a 64bits environment you get the error 51. this is because the Cisco VPN version is 4.9.01.0180. It looks like it crashes even you shut down the fw0 and restard the process with ‘sudo SystemStarter restart CiscoVPN’ . I tried with the last version provided by Cisco the 4.9.01.0280 and it dosn’t work either. It’s so pity the war between Cisco and Apple!
The only thing you can do is to downgrade to 32 bit system.
1) For current stratup only
On startup keep holding the 3 and 2 key and your system will reboot on 32-bit kernel and press 6 and 4 key for 64-bit kernel.
2) For persistent
Go to the termianal and use the foollowing command
for 64 bits => sudo systemsetup -setkernelbootarchitecture x86_64
for 32 bits => sudo systemsetup -setkernelbootarchitecture i386