I haven't downloaded the "a" update yet but it works if you save the custom field first and go back in and add the condition fields. Same for a copying field. Add new or copy, save it, and then you can go back in and add the conditional field. Just ignore the error at the top. I suspect the latest download will take care of it.
Yes, there are some issues that came up... mostly tied to IE
We have now updated the package again after addressing further items that have been discovered. Please download the package again and reinstall. The updated package will show installed as 2.6.6b. You will find radio buttons also working for conditional fields now. Checkboxes work for conditional fields also, just not in IE. That will be fixed by tomorrow at the latest.