The alignment for the donation levels can be adjusted either in an HTML caption for your donation form, or in the Page Wrapper. Use the following CSS to align the donation levels:

<style>
div.donation-level-container {float: left;}
</style>

"left" can be replaced with "center" or "right", depending on how you want them aligned, but if you have a lot of empty space, then it is probably because it is not aligned to the left.