Scroll down the page to find the logo you want. Logos are offered in two formats: png (better for onscreen viewing such as Internet and Powerpoint), and jpg (better for printing — documents such as Word).
To save to your computer, simply right-click on the image text link of your choice, choose "Save Target As...", and choose a folder on your computer in which to save the file. (Mac users, click and hold on the text link and select "Download Link to Disk.")