To create PDF forms using PDF-XChange you will need either PDF-XChange Standard or PRO Versions and have selected to install the PDF-XChange MS Office Addin.
The best place to start is to read your MS Word Help file regarding the use of Forms Fields within MS Word and specifically you can only use MS Office 'Web Form Fields' in your document in a PDF form.
Once you have created a Word document that conforms and uses Web Forms Fields as defined by MS Word - then you can simply convert such a form compliant document by selecting the PDF-XChange Icon on your Toolbar for 'Settings' and ensuring that the 'Forms' TAB on our settings options has the appropriate 'Forms' Support option checked.
From the MS Word Help file:
You create a Web form in the same way you normally create a Web page, and then you use the Web Tools toolbar (toolbar: A bar with buttons and options that you use to carry out commands. To display a toolbar, press ALT and then SHIFT+F10.) to add form controls (control: A graphical user interface object, such as a text box, check box, scroll bar, or command button, that lets users control the program. You use controls to display data or choices, perform an action, or make the user interface easier to read.). Microsoft Word provides several standard Web form controls. When you add your first control, Word inserts a Top of Form boundary above the control and a Bottom of Form boundary below the control. These boundaries appear when you design the form and will not appear when the page is viewed in a Web browser (Web browser: Software that interprets HTML files, formats them into Web pages, and displays them. A Web browser, such as Microsoft Internet Explorer, can follow hyperlinks, transfer files, and play sound or video files that are embedded in Web pages.) You can add other controls to the form by placing them within those boundaries. You can have more than one form on the same Web page. Each form exists between its own boundaries.
MS Word even provides example Web Forms for you to copy and modify to help get the hang of things