You can find it in PRODUCTS module of your administration panel.
If you successfully added size charts, all you have to do is to assign them to products, so the system could know, which one of them should be displayed to store visitor. To do this, go to product editor in panel, find "Size chart" option and click on a magnifying glass next to it. Select size chart and save changes. Now check product card in store - next to a list of product sizes a size chart along with measuring instructions should appear.
Size chart displays only products, that are visible and possible to be chosen by client on product card in store. If browser product does not have a size added to size chart, this size will not be displayed in store.