Associated subgrid on entity form has changed significantly since CRM 2013’s new clean UI.
There is Add (+) button above the subgrid at the right hand corner which replaces the Add New and Add Existing button which normally appear on subgrid Ribbon control for CRM 2011.
Having the same button performs two different actions is tricky and can sometimes be confusing for user because of the different UI interactions when user clicks on the Add (+) button.
Fortunately this behavior can be configured from the relationship’s (one-to-many) required attribute:
A. REQUIRED, the Add (+) button will launch a new form (Add New).
B. NOT REQUIRED, the Add (+) button will let user search from existing records (Add Existing).
Option A: use this when user normally creates new related record, eg. create new Opportunity from an existing Account.
Option B: use this when user normally searches for existing related record, then creates a new record when no match found, eg. add related Case(s).