I've added a "login" button, and toned down the secondary X. The user name and password boxes were shortened a bit to make room for the button.
I was concerned that the second White X was competing with the X in the main logo- What do you think? If you think it was better with the white I'll change it back...
Try making the button match more with a dark blue background and white text. Also, try skewing it so that it goes with the style.
I think if you designed the base image as a vector shape and added two anchor points and expanded to create a curve (firstly skewing it of course) this alone would give it a nice shape. Just a thought.
Example: Something like this (ignore the colouring)