Let's see if I can help you out. A few questions:
It sounds like you might be trying to attach a file while authoring the form on the Design tab in FormsCentral. Is that what you are trying or are you filling out the HTML web form and trying to click "Select File" there?
If you are trying to attach in the Design tab of the form that is not supported, you cannot attach files to the form as the author when authoring, the "File Attachment" is for when users are filling out the form so they can submit an attachment to the author.
Thank you. I was in the Design Tab and trying to attach a form. I thought I could attach the conference agenda. Thanks for clarifying.
Meeting & Event Specialist
A & E Events