Posting the error message usually helps.
Which I see you have done now.
Have you hit the Citrix support site?
I actually got that error while trying to run it over safari. I use firefox for my citrix login now.
Ah, I actually knew that one.
FWIW, it's not actually a problem with Mac OS X or Safari. It's lazy coding at Citrix. They don't want to have to support Safari, so they BUILT-IN limitations that check for Safari and (basically) fail it. You may find that if you use the Debug menu in Safari and pretend to be IE or Mozilla, the site works as normal.