Out of the box, in webcalendar events the "Insert" tab of the Aloha toolbar doesn't have the option to insert library items (at the time of posting this applies to versions 8.6, 8.7, and 9.0.) This is how to add the "Insert Library Item" button to the toolbar for your CMS users.
In the file "ektron.cms.framework.ui.config" in the root of your site folder, you will add a value of "ektron/library" as demonstrated in the line below. You will want to put it (comma delimited) between "common/contenthandler" and "common/block." Please also see the screenshot highlighting the change.