VBox maximize problem on Ubuntu

I'm using Ubuntu 11.04 on VBox v4.0.4. My problem is, I can't maximize the ubuntu windo开发者_运维技巧w. It normally just stuck at the center of the VBox.

I already tried installing guest additions then restart but no luck. It doesn't worked. Any ideas? Thanks!

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Press Host+F key for full screen and press the same combination again. Now resize the window and click on Maximize button.