How do I get a bottom to top build in animation with Keynote?
If each section of the pyramid is a separate object then this shouldn't be too difficult to do via the inspector. All you need to do is select each object one at a time and add an entrance effect to it. Then in the animation tab of the inspector you can click on "More Options" (in the same place that the "Close Drawer" button is in the screenshot below)  and a drawer will slide out that allows you to change the sequence of animations (by dragging and dropping). Here's a screenshot of a simple sequence involving three objects. 



It is also possible to control the animation of text boxes in a similar fashion (since they can be selected as individual objects). Additionally one can also set bulleted text to come in one bullet at a time with the following settings:



So instead of setting the "Delivery" occur "All at Once," change it to "By Paragraph."