Under Discover in the app menu, you can type the title or composer of the piece you're looking for into the search bar at the top.
Filter by instruments
On the home screen, tap where it says "Filter" near the top middle. Here you can select how many parts you want the search results to contain, by tapping the + and - icons to adjust the number, and select which instruments you want the scores to contain. When you've made your selection, tap the < button in the top left to apply the filter.