You should add your certificate here (your capitalization may be different in the highlighted section):
Then load the certificate with this command:
cisco_cert_mgr -U -op import
You should be prompted for the filename and a password. Then, verify it is showing up in the store:
cisco_cert_mgr -U -op list
Connect to the profile (you may need to set this up if you haven't already, check that out if it doesn't work at this point) with the certificate and you should be good to go.
I realized that I left you hanging at the end...in case you don't have a profile set up, you'll need to set one up. Check here for profiles:
You will need to edit a profile or create a new profile and it should point to the certificate. The following settings are relevant to this task (change the red text):
AuthType = 3
CertStore = 1
CertName = Name
Finally, activate it with this command, change the profile_name to whatever profile you just set up but leave off the .pcf extension:
vpnclient connect profile_name
NOW you should be good to go!