Centering Windows on screen

You could use [window center] as answered by wahkiz, but that might not be exactly what you want. The documentation states that this will put the window centered horizontally, but somewhat above the vertical centre. In this way the window has a visual prominence.

If you want to put your window in the exact centre, then manual positioning (as commented by Koning Baard XIV) will work for you.

Use [window center]; for the centre of the window to be centred to the screen. Also, assuming you're in 10.5+, there is a centre to screen button in Interface Builder.

In your appdelegate, in applicationfinishedlaunching, add this code at the bottom.

[window center];