The reason it is offline is that the Mac cannot see it. And that makes sense given that when you did try to add it again, the MP500 was not shown in the Default Add Printer view.
And all you have done by following that other thread is to return the proxy for the MP500 to the printers list. But if the printer is not seen by the Mac as physically connected to it then it will be offline when you try to print.
So you need to determine why the Mac cannot see the MP500. The easiest way to determine this is to use System Information, which is located in the Utilities folder in Applications. With System Information open, in the left column you will see USB. Select this and the top right pane will show all the USB busses on the Mac and the devices connected to them. The MP500 should be seen here under one of the busses. If it is not, then that is why you cannot add the printer and why the current offline status is appearing.
If the MP500 is not shown then with all that you say that you have tried, ie another USB cable, another USB port on the Mac and powering the MP500 off/on, then it would suggest that the USB port on the MP500 has failed, which is not common but can happen.
If however the MP500 is seen under one of the USB busses but you still do not see it when you select to add it via Print & Scan, or the MP500 you created via the other thread still shows offline, then it could be a case of current draw. With the MP500 selected in the USB section of System Information, the bottom pane shows information about the connection. Copy and paste that information back here please.
And for the record, the printer is supported. Canon provides the driver to Apple so that is can be downloaded via Software Update.