Open the document in Acrobat DC. You should see a number of Tools on the right side. Among them is "Prepare Form". That's the form editor. You can also get to it by clicking on "Tools" at the top of the window, then either look for the "Prepare Form" item, or just type "form" into the search field.
As to why the document is not working, I would guess that the most likely scenario is that your users are using a web browser with a built-in PDF viewer (Chrome and Firefox). For Firefox, you can actually make things work by disabling the built-in PDF viewer following the instructions here:
http://helpx.adobe.com/acrobat/using/display-pdf-browser-acrobat-xi.html
For Chrome, the situation is a bit messier: Chrome decided recently to no longer support browser plug-ins like the Adobe PDF viewer. This means that unless you save the document to your local computer and open it in Adobe Acrobat or the free Reader, your form will not work.
You will have to require that the document is edited in Adobe Acrobat or the free Reader, otherwise your form functionality will not work at all, or be very limited.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com