The error 51 occurs because the software is a 32-bit application and you are trying to open it in a 64-bit environment.
To resolve this you have these options:
- Install Shimo: This is a very complete software. Also has full support for Cisco and many other networks.
- Start OSX in 32-bit mode by pressing keys 3 and 2 on boot. Unfortunately this is possible only for Lion and earlier versions. Mountain Lion only runs in 64-bit mode.
So, if you need to connect to a Cisco VPN every day like i do, the best option is to install Shimo.