When you open the Fltplan app, click the "Approach Charts" icon
You should have 3 tabs on this page: All, Favorites, and Recently viewed at the top. This is where your approach charts will be stored.
In order to download the plates that you require, tap the download icon at the very very top right of the screen on that page. It's a down arrow with a rectangle shape underneath it.
This brings you to the "Download Manager". Tap the area of plates you are looking for. Beware tapping the big green download button here as it will download all the plates for every state in that region. If that's your goal, then this saves you a step. If you'd like the plates for a specific state or set of states, then tap the region (i.e. southwest, eastern, etc.) This will bring up the list of states within that region, and the details of the plates. Tap the state of plates you wish to have and it will bring up a download sidebar.
This will show all active downloads. You can download multiple states or regions at the same time, so don't feel like you have to wait for every download to return to the Approach plates page to get more.
You can also download pertinent maps and documents in this way from the download manager.
Hopefully that answers your questions.