How to show messageBox with InstallShield 2009

Question

I'd like to show a messageBox in the setup.

Have how to put this messageBox into a Custom Action? Because I want to this messageBox appears when one condition is satisfied.

If isn't clear, please, let me know!

EDIT:

I want to make a script to verify if the .exe that I'm trying to execute it's already installed, if don't --> execute, else --> do nothing.

Answer 1

Use an InstallScript function, there you have a MessageBox().