You MAY distribute it and charge for that service. You MAY change it, add design and content to it and you MAY charge for that. You may NOT alter the license and you must NOT alter the copyright. You do NOT have to show a 'Powered by Mambo' graphic, as it not a copyright notice.
In other words, you must NOT pretend that Mambo is yours, and you must NOT charge people for Mambo. Use Mambo to empower yourself and your clients by taking away the 'Black Magic' that surrounds putting content on the internet. Charge for the value you add and not for the hard work that Miro, the Mambo Development Team and the Mambo community have put into it.