Can't scan from HP psc 2400 to iMac computer

You need to set up the software on the iMac. The reason it gives you that error message is because it can't communicate with the computer and imaging software as the computer doesn't see the scanner and can't communicate with the printer's control interface. The iMac may see the printer and be able to print without any additional software because the driver package for the printer portion may already be installed with the OS X HP printer driver support. You didn't mention whether or not you could print from OS X. I am assuming you can. Faxing and copying from the unit doesn't use the computer at all and doesn't need to communicate with it. Scanning and printing do. If you can't print either then none of the driver or support software is installed and you have to set it up. You can get the instructions for setting the printer up in OS X 10.5 from HP. Just click here and it will take you to the page with the instructions.

Best wishes and cheers