Click the drop-down arrow to the right of the calendar you'd like to add. Select Share from the drop-down menu.
Check the box labeled Generate links (most secure).
Right-click the first URL under To import into a Calendar app (ICS).
Copy the calendar URL by clicking the Copy link address command on the popup menu. Depending on your web browser, this command might be named Copy shortcut or Copy Link Location.
After copying the link, the address field at the top of the Add Calendar Source - Yahoo! Calendar window should be automatically filled. If that doesn't happen, right-click the address field and select Paste from the popup menu, or press Ctrl+V.
Enter the name of your Yahoo! Calendar in the field below the calendar address field.
The OK button will now be enabled. Click the OK button to add your Yahoo! Calendar to PrintableCal's list of available calendar sources.