In Salesforce, add-ons attached to subscriptions are presented as Recurly Line Items. If you want to be able to see this information, you will first need to enable the syncing of line items from Recurly -> SFDC. To do this, go to the Recurly Admin tab and from there, select "Advanced Settings." Scroll down and check the box for "Line Item Sync Enabled." Once this is turned on, you'll see a new section named Recurly Line Items under the Recurly Account object, with add-on and other line item data.