The latest release of RSVPMaker addresses a longstanding request for an easy way to specify that certain fields on the form should be required. See this demo event with required fields.
Using the shortcode-based template system for the forms, I added my custom field in this format:
[rsvpfield textfield=”extrarequired” required=”1″]
Formatting of the error message is controlled by this addition to the plugin CSS (which you can override by specifying your own CSS in RSVPMaker Settings).
If someone wants to volunteer to make this prettier, let me know, but at least the basic function is there now.
BTW, one of the recent releases of RSVPMaker had some coding errors in the default version of the form. These are corrected now, but it’s possible you have the botched version saved on your system. If you look at the Documentation screen on the RSVPMaker menu, there’s a link at the bottom you can use to reset the form to the new, improved default. If you’ve fixed this yourself already, never mind, and I’m sorry for any trouble I caused.