Welcome to the discussions forum MrMom.
I think your Mac ought to be running OS X 10.4.
11, so check Software Update again. If you're not successful, try this link:
http://support.apple.com/kb/TA24901 and select the *Combo Update* for your iMac (PPC or Intel, as appropriate).
HP's software for the PSC 2510 is located here:
Select software and drivers.
Click on the link "Software for HP PSC 2500 Series - Mac OS X v10.3 and v10.4 (Universal)". I believe that's what you'll need.
According to HP and my own personal experience, the
print drivers for this printer are already built into OS X. You only need the above software to use the scanner and fax features. I have a reference to that effect at home but I'm not there right now, so if you don't meet with success let me know and I'll research this further.
Seems that my wonderful all in one printer is obsolete and the company has practically disowned it--no updates, parts, advice for this item.
HP is not the company it used to be. Their interest in, and support for OS X is extremely limited. As such HP has tied themselves to a dying market. I no longer recommend them.