Re: how to install apk files on android lollipop?
Welcome to Android Central! First, is there are reason why you're trying to install apks instead of installing from a legitimate app store like Google Play Store or Amazon Appstore? The sites where people can get apks are often good places to get malware as well, and the apks are often pirated (which we definitely do not condone here).
If this is a legitimate apk file (e.g., the Amazon Appstore installer file), then make sure Unknown Sources is checked on in Settings>Security.