This is a sample custom action that shows how to update the action text and progressbar. It contains a sample.aip file, that calls the custom action, which you can build even with a trial version of Advanced Installer.
The DLL "ActionTextandProgressUpdates.CA.dll" is a standard MSI custom actions DLL, so you can use it in any installer package, be it built with Advanced Installer, Wix, etc...
Attached you can find the solution built with VS 2010 and a sample project file built with Advanced Installer 10.7.
If you have any questions let me know.
P.S. One MSDN you can find alternative C++ sample and detailed explanations about custom actions scheduling.