Resolve Field is free to download from the App Store. It's built for iPad, so you won't find it when searching on an iPhone.
Open the App Store on your iPad.
Tap the search icon and search for Resolve Field.
Look for the orange app icon with the white Resolve logo. The developer should be listed as Resolve BIM.
Tap Get to download and install the app.
Once it's installed, open Resolve Field and sign in with your Resolve account. See Signing In to Resolve Field for more details.