I just ran into this problem.
Don't try to create a site with a "shared login" like Google or Facebook or your brain will explode in pain and torment.
You should go to their website create a REAL account using your name/email. This is what you need when you use the plugin with WP.
If you use "auto login" from Google, FB, etc., you run into all sorts of bugs, like the system auto-logged me in with my FB, even though I have a login and had entertered that, didn't realize it had done that created a site, then it wouldn't let me delete it as you need a password entery to delete a site. So, if you are auto loggged into an account via Google or FB... how could you enter a password to connect with WP, or delete an errant site ?
There is no "barrier" between the "for sites" user and a registered commenter, apparently -- so ... even if you try to login with your actual admin username/pass, it will create new account for you if it's tracking you via a FB or auto login. Doh.
TRUST ME: create a real account ... or prepare to meet your doom. There is no support contact on their site, so it's pretty much a fluster cluck unless you create a real account THEN make sure you are logged out of EVERYTHING when you go to their site to admin anything.