InstallScript provides a built in SelectDir() function, allowing the user can select a directory. However there is no equivalent function to allow the user to select a file. Which is a problem if you want to implement custom dialogs that browse for files (e.g. a dialog that asks for a .pfx certificate file and its password).
As it happens, InstallShield provides a very good knowledge-base article about how to add such functionality, but it is very-well hidden; you have to use very specific keywords in order to find it… So, to save you some time – go ahead and use this link.