Hi there,
It's vital that you're able to use the program without running into issues. Don't worry, I'll be happy to look into this and give you a hand.
I'd like to start off by doing some troubleshooting to determine if this isn't a browser related issue. When was the last time you cleared cache and cookies on your browser? Doing this will help fix most issues the site may be having. Here's an article which shows you how to do that: How to clear Cache and Cookies. Give this a try and let me know if you're able to add the categories.
If you're still having issues, feel free to send me a screenshot of what you're seeing on your screen. I'll be here in case you need further assistance.
