Atari 2600 emulator help?

I have Stella, whever I start combat, the screen just sits there and the colors flash to different shades, and it just keeps doing that.

My p0 joystick is mapped to the arrow keys and space to fire, but no matter what I press I can't get it to do anything.

Help?

edit: screenshot: http://i.imgur.com/MBUSY.png

Well first off this is how the atari works. You need to hit select a few times than start or something. I believe the buttons are mapped to f1 and f2 keys. I normally hit f2, then f1, then the fire button.

Thank you so much. That works perfectly.