PMFCreator for cintro
Its nice and easy to use: Just run pmfcreater.exe to start the gui. Browse to your MPS, no need to rename it. Then browse a save location, name your intro as intro.PMF.
You will also need to enter a time length, the length of your intro in minutes and seconds. You can only use whole seconds though, so if your intro is 9.5 seconds, enter it as 10 seconds in pmfcreater.
Hit the convert button, a confirmation message will pop up when its done. Copy the intro.pmf to your PSP\SYSTEM\INTRO folder. You can preview your custom intro from the xmb with the PMF Player Test App, which comes with the CIntro download.