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 in the list. The AIML will be converted to Self, so if you click on it, and click the edit button it will show the Self code.
Check the log from the Log tab for any errors.
Another option is to import it as a chat log from the Chat Logs tab. This is better for large files, as it will treat the AIML like any other response data, and integrate it with your bot's existing responses, and any future responses you train it with.