ecwid is integrated through CAS. To set up ecwid, go to Configuration>Integration>ecwid
Once logged in, specify how inventory will be handled, and then accept terms and conditions. Determine if the inventory sold in the ecwid store will be taken from a store or warehouse, or if it will not have a shared pool of inventory. If not sharing inventory, set up a store and then create a register with a web license for this store.
If Yes - you want to share an inventory pool, select which store or warehouse to pool the inventory. To proceed, accept the terms and conditions.