1. What is the email address of the Gtalk account?
2. Are you online and listed as "available"? (if you are listed as busy or away, we will show you as offline)
3. Do you see "helpdesk" in your buddy list? is it online?
4. Do you see any of the "visitor.slot" or "webuser" buddies from Olark? (you might need to check your Contacts and do a search for "olark")
Also, in order to properly connect Olark live chat to Google Apps for you domain, you or your ISP must configure your SRV records (part of your DNS records that pertain to extra services like chat) to allow external services like Olark to connect. Check out our SRV walkthrough
One other option: if you continue having difficulty with Gtalk, we might recommend using our Olark Chat OpConsole. The overall experience tends to be much better, if you don't mind using a web-based client. You can use iChat on Mac (view the tutorial) and Pidgin on Windows (view the tutorial).
Hope this helps!