I wonder if what you are doing is in fact searching in existing links only?
There are at leat two ways you can use to link an existing appointment to a contact:
1. In calendar view, right click on the appointment and select New > Link. Search for the contact and link it.
2. If you double click the appointment (as to edit it), you can use the "Link" button. But then you will have to click the other link button (that you find in the newly opened window
It seems to me that you have only done the first part of #2, and that you in fact are searching only in (non)existing links....
I am new to GroupOffice, is there phone support or an actual user training manual to learn this system and all of the features? I know we are under-utilizing the power of this CRM, because no one has been trained on it. Sadly, I believe we will need to change systems, because of this issue among many others that we can't seem to find answers to.
If you have the PRO version the company behind GroupOffice will give you directely access to their support ticket system. They stand behind their product 100% and will go a long way to help out with any problems.
The GO Wiki is a good place to start for learning GO. Click Help > Help for the Wiki.
If you are using GO in a company environment you really should get the PRO version. In some countries you can also find GO resellers that offer support and training.