The scan software is not supported on your OS. Your printer model is too old and HP will not likely go back and make a new version for your OS if they havent already.
You must use 3rd party scanning software if you want to use the scanner. Apple should have a free imaging application you can take advantage of. There are also other scanning applications out there for free and for cost. Look around and find one that fits your requirements.
All you need are the TWAIN drivers which can be aquired through Apple Updates.