The Mac Cisco VPN Client can only run when OS X is booted into 32-bit mode. Cisco has no plans on making it 64-bit compatible.
Leopard (OS X 10.5.8) and Tiger (OS X 10.4) only support booting into 32-bit mode.
Snow Leopard (OS X 10.6) boots into 32-bit mode by default on most Macs… with the exception of all Intel Xserves and Mac/MacBooks Pros manufactured after 8/2010, which boot into 64-bit mode by default.
Lion (OS X 10.7) boots into 64-bit mode by default on all Macs.
Both Snow Leopard and Lion can be forced to boot into 32-bit mode. When booted into 32-bit mode, the Cisco VPN Client works fine.
Mountain Lion (OS X 10.8) only support booting into 64-bit mode. This means the Cisco VPN Client is not compatible with Mountain Lion.
Mountain Lion can be configured to natively VPN into Cisco ASAs that are configured to allow connections using the Cisco IPSec protocol. This is done by adding a VPN interface in the Network system preference pane.
Many Cisco ASAa are configured to only allow VPN connections using more secure protocols, including most of the Cisco ASAs that Techcare (now All Covered Chicago) has configured over the years. This means connecting via the IPSec protocol isn’t an option without changing the ASA’s configuration.
If you need Mountain Lion to connect to a Cisco ASA that isn’t configured to allow IPSec connections, and making a change to the ASA isn’t an easy option, then I would recommend purchasing the third-party application VPN Tracker. The Personal version is $130, and supports directly importing Cisco pcf files, making it as easy to use as the Cisco VPN Client.