I can see the button in my Chromium, right below the SSO buttons. Please make sure you've followed the steps in here, specially the flags to use when starting chromium:
There was one time I couldn't see it though, and it was because of a timeout (the server took too long to respond). When you enter the site URL, the app performs 2 requests: one to check if the site exists, and another to check the site configuration (to see if signup is enabled and so). If the second one doesn't answer in time (30 seconds), we still show the credentials form but we can't show the signup because we don't know if it's enabled or not. We aren't blocking the user in this timeout because some sites don't have the required WebService, so we could be blocking the user for no reason.
I tried it like 10 times and only 1 failed, the rest of requests returned really fast, so it seems to be a weird case (at least for me).