I just tested this, and one possibility, one thing that could affect that button is if you have something called "NoScript" installed on your browser, which is a privacy-oriented extension that disallows JavaScript to be run in your browser without providing explicit permission for any sites to do so. I have it installed on mine, as I prefer my permission to be explicit in matters of privacy but gave permission to PC so that all the features work.. but I turned it off for a minute just now to test it out for you, and it did cause that button not to work.

So if you do have NoScript installed you just need to right click somewhere on a blank part of the page, click "NoScript" in the menu, and then click "Allow PsychCentral". (Not "Temporarily Allow PsychCentral" or it will only work for one session!)