Currently the 0.9.0 version is in development, but you can use 0.8.2. I just tested it to confirm it is working. Here are the steps/commands that I used, please use the same:
git clone https://github.com/graphhopper/graphhopper-ios
git checkout 0.8.2
git submodule update --init
git submodule update
Now, open the Xcode project at graphhopper-ios/graphhopper-ios-sample/graphhopper-ios-sample.xcodeproj , make sure the "graphhopper-ios-sample" target is selected (instead of the "graphhopper" one) and hit Build or Run. The project will build successfully.
Keep in mind that the sample app is using an older MBXMapKit, so the map and the drawn route are just for demo purposes, and bugs can be found. However, a basic example on how to use the GraphHopper library can be found in Directions.m.