You can display fields for data entry on a custom dialog only as Edit Boxes No radio buttons, no checkboxes, no pop-up menus, no drop-down lists. If you set a field on a layout to show values from a value list display as a drop-down list, these options will apply to that field only on that layout (not another instance of that field on any other layout nor in a Custom Dialog Box). If you have a value list assigned to a field for validation in the Define Database dialog, then that will of course hold for any instance of that field on any layout and in a Custom Dialog, although this will still not make that field in a Custom Dialog pop-up.
The inability of the Custom Dialog to display anything but Edit Boxes, along with its inability to display more than 10-20 of the 32 characters it allows you to enter for button captions, and its refusal to resize itself automatically to show a text message more than a few lines long, severely limit its usefulness.
You will want to make your own Custom dialog; this does NOT use the Show Custom Dialog script step. Rather, you create a layout with all the pop-ups etc. you desire, then your script would open a new widow showing that layout, pause for the user to pop away to her heart's content, and resume when a button scripted to resume script is clicked.