Hi Heather,
I have tried to replicate your issue but on my end, going to Customize -> Tables / Fields -> Add a new Field works properly.
Could you kindly go to Customize -> Users and enable the MethodSupport account. I will jump into your account and double check to see what is happening on your end. Please also email me the name of your account that you use to sign in so that I know what account is experiencing the issue.
Regards,
Denis