In order to add Google Ads Line Items to Campaigns of an Ad360 Advertiser, you first need to link this Advertiser with a Google Ads Account

The list of available Google Ads Accounts you can link will be displayed in the drop-down menu named "Google Ads Ad Account". 

To be included in the list, the Google Ads Account must have been linked in the Ad360 Account settings first. Plus the Google Ads Ad Account needs to be active.