There are different ways in which you can install an app on BlueStacks 2.
Watch this video to guide yourself how to download and install apps on BlueStacks!
Watch this video on youtube!
Kindly follow the steps below to install an app.
 1)  Search and install: On Home screen, click on the play store icon as shown below:
Type the app name which you want to download in the field provided.
Click on the "INSTALL" button to install and enjoy your app.

2) Drag and drop: You can now drag and drop .apk file of any app on BlueStacks App Player. All you need to to drag .apk file ( present on your system) and drop it on App Player screen. It should start installing the .apk file. 
3) Apk button on sidebar: You can now install any .apk file directly from the APK
tab available on the sidebar. Clicking on this button will let you install .apk file saved on your PC.