Login for both http & https

i have installed Simplemodal login for my site at http://www.instantvietnamvisa.com
i can login normally with http url (http://www.instantvietnamvisa.com/) but i could not login with https url for example: https://www.instantvietnamvisa.com/apply/ and reverse if i use Force SSL Admin
please advise.
thank you.

http://ww.wp.xz.cn/extend/plugins/simplemodal-login/