Untappd for iOS and Android are pretty much the same, but let's start with the basics: In order to check-in (which is the term used to record a beer on your profile) to Untappd - you need to search for that beer. Here is how you do that:
Tap the beer search icon on bottom navigation (it's located in the middle). That will take you the beer search page. From here you can search in the upper search bar or use the barcode scanner to find your beer to check-in!
Tap left menu that will bring out the sidebar which contains a search bar.
Hope that helps!