how to get rid of installation alert sign on mac?

Finder > Applications > Utilities > Activity Monitor

All Processes > CPU

Select iTunes helper.

Click the X button at the top.

Confirm it.

Also note that if you have multiple users logged in and another user has the applications open that the installation alert is referencing then A)that may cause the alert in your account and B)quitting those applications in the other users account may remove the alert. In my situation my wife and I use the same laptop but have separate user accounts and I found that when I got one of these alerts, logged in to her account and quit the relevant apps, then logged back in to my account the alert was gone.

Gonzo is absolutely correct - I had another user whose account had iTunes open. Closing that down got rid of the installation alert window and the install to continue.

In my case the installation alert still appeared, although iTunes was closed on every account. The problem was my iPhone being still connected to my Mac via USB (even if iTunes was not opened). So make sure that all mobile devices are disconnected, if the previous answers have not solved the problem yet.

My whole family uses this Mac, when up to two people are logged in it goes really slow. Then just today the Installation alert popped up, all I had to do was logout of my mom and my sister's account and tada, all gone. This could also happen if something like skpe is open, just eject it on one of the accounts it got downloaded on and logout, all gone.

Nope. This does not get rid of the Installation Alert in my case.