First, make sure you are logged in, browse to your bot, and click on its Admin button.
On the Program tab the upload/import button is the last image button on the right. When your mouse hovers over it, it will display some import options like in the picture below. It defaults to "Self" so you need to select "AIML" and then click the upload button and select the file.
Depending on the size of the file, it may take a while to import. Once imported, it will show up as the last script