SSL error on Mac Pro

I have recently bought a Mac Pro octo. My husband works from home (old machine was windows vista ultimate). He using citrix to access his works servers and see the necessary information for a days work.
Unfortunately i am getting an error when i launch citrix :

SSL error 61
You have not chosen to trust “essential SSL ca” the issuer of the server’s security certificate
Error 183

I get this error everytime i try and sign on to citrix. I have reinstalled my operating system twice just in case any of my tweaking has disturbed something i needed. The certificate is already within my machine I am told when i try and import it. Is there some sort of work round to enable mac and citrix be compatibie. I have changed securities. The person who deals with the servers at my husbands work sent me a link to the certificate download but it didnt work… assume it was an old link. As far as I can see I have done everything I can.

Can you throw any light on the situation.

Deborah Robinson.

I Would Like To Know Whats The Version Of Citrix Client Version You Are Using ?

If i understand you correctly it is the one on the citrix website version 10.00.063 - is this what you required/

What type of certificate are you trying to install? Is this the free SSL certificate? When trying to sign on to citrix, what url is used?