If you a have multi-issue app (Mag+ Grow!) you don't use iTunes Connect to create your free issue. All you have to do is to add a new issue at publish.magplus.com, then when setting it up, select it to be a "Free issue". The issue will then show in the library of the app, to point the user to download the free issue in the library we recommend you to use the banner feature that you setup under APPS > Live Settings > Banners at publish.magplus.com.
Let me know if there are any problems!