After you have downloaded and installed ThickButtons you need to enable it as an input method.
To do so go to Settings.
Choose Language & Keyboard.
Activate 'ThickButtons'.
Warning message will appear, press OK to enable ThickButtons input method.
Tap and hold for awhile any text field e.g. Search Box. 'Edit text' menu will appear, select Input Method menu.
In the 'Input method' menu select ThickButtons.