Which iOS version you are running? Unless its below 6.1 it should install with no problem.
6.1 is there because I did not modify default XCode setting - in upcoming update minimum system version will be set to 5.0
Anyway - If you can't get it from AppStore check and possibly update your iPhone. Unless you have it jailbroken of course ;-) In that case just wait for an updated BTC Miner v1.1 (approx. next week it should be out).