Grid-focus help for newbee :(

I am running grid-focus.

i am stuck with some things I want to achieve.

How can I extend the size of third column?

currently it is like this,

http://imageshack.us/photo/my-images/191/columnsize.jpg/

I want to extend it similar size to the edge of the header.

can you help me as to what I should do? I am a beginner.