When you plug your iPhone or other iDevice into iTunes and try and drag and drop apps to your device, sometimes the apps are gray and will not let you select themselves to add to your device. Follow these steps below to fix this issue.
Step 1) Open iTunes and select your device on the left sidebar.
Step 2) Go to the Apps section.
Step 3) Select "Sync Apps."
Step 4) Check the apps you want to add to your iDevice.
Step 5) Select "apply" in the bottom right hand corner.
Step 6) The apps that you just selected should now be enabled to drag onto your device!