Hi,
A link to your website would be required to help you for this kind of CSS customization.
It could be quite simple to fix but your problem is link to your template. I guess that "submit input" buttons should have a "float:left" css rule.
A
#hikashop_checkout_login_left_part .hikashop_cart_input_button { float: none; }
Should fix your problem but, the button would be placed on his original place : between "remember me" and "forgoet your password".
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.