When you attach items to your events you can either attach items that have already been uploaded to your account (like addresses or files or map links) or you can attach something directly to your event when you create it in your calendar. You can directly attach addresses, files, photos, discussions, and map links.
To attach items directly to your events you'll first need to open the Event Editor by clicking on a blank space in your calendar. Then you can click on the "More Options" button and then click on the "Upload" icon next to "Attach Links." Follow the steps in the image below:
Once you've chosen what you want to attach to your event then you'll either add your link or choose the file from the computer you want to upload. Then you'll see a confirmation dialog listing your new item attached as a link to your event. See the image below: